Inspired by comments and answers to Do professors normally explicitly state on their web page that they aren't taking new applicants?
Given that most professors' websites are years out of date, empirically ...
Why are most professors' websites years out of date? What is stopping them from updating their websites?
As an example, my "official professorial" web page is on my college's website. The institution formally controls what's published there, such that any change has to go through multiple layers of administrative oversight, approval, handoff to technical staff, ticket pipeline -- and then likely iterations on fixing errors that pop up through that broken-telephone game -- before being actually live on the site.
It's so much work, and uncertain to actually succeed, that many staff and departments just give up and don't bother except in cases of really huge overhauls. Frequently by the time you get around to making a change, the whole process has been reorganized, and then you need to add in some advance research to find out who the current lead contact is, what the process is, how they expect the change to be communicated, etc. I think if I were to ask my department staff right now, they wouldn't know how changes can be made.
Over time, anything that feels like PR gets more and more tightly-gripped by the college administration, and therefore harder to change, I'd say.
Various reasons:
Sadly, course content is becoming a walled garden. When I was a student, many professors had a page with their own course notes; today the same course notes are more often hosted on a Google Classroom, Microsoft Teams class, or closed Moodle instance, and they are not accessible from people outside the university. I see this as a big loss.
The incentives in having a personal website are typically tied to hiring. A website is a business card; once you have tenure, you are swamped with other responsibilities and updating your page becomes less important.
Another thing worth pointing out is that today having a website is the exception, not the norm. Which other category of people has a personal website, today? I mean a true self-hosted website, not just a social media page. Programmers, mostly, but it could be argued that for them it's also a business card to get a job; it just gets updated longer because they job-hop more.
The other answerers have covered most of the reasons. Elaborating on two problem areas that I find personally compelling:
University administrators and history of this technology.
I started maintaining a personal web page around 1995. I was quite excited about it, adding material as I saw fit. Quickly started using it as a channel for distributing information and material related to my courses as well. The pages were crude (basic HTML of the era), but they were A) easy to maintain, and B) got the job done. Many colleagues in Computer Science, Physics and Math did the same (maybe STEM fields were quick to adopt this). One could learn enough HTML in a few hours. Over the years I added more and more stuff. After a 3 hours course on Perl I even added a crude Q&A window on the course page, but the students were not really interested, and they disallowed CGIs (the first blow). But it has gotten progressively worse.
SCREW THEM!
TL;DR;
I don't feel great about posting a rant like this. You can probably guess that the underlying sentiment has been brewing for a long time. Judging from the votes I'm not the only person feeling this way.
In the interest of fairness I would welcome an answer/comment from some university web administrator (or some such). Admittedly this answer may not be very conducive to a dialogue like that. But lack of dialogue from their side made the problem significantly worse than it needed to be. Maybe that ship sailed :-(
Effort and reward.
Effort: Updating a website takes effort. Maybe a lot of effort (see Daniel's excellent answer), maybe a little less. But not everyone knows how to update their site, and there's no real reason to expect them to know how. Why should a professor of (say) history know HTML code and so on?
Reward: There isn't much. What does a professor gain from an updated site? Probably what changes the most often is publications and that can go on a CV, or, for other people who want to know what a professor has published, there's always Google Scholar.
Professors get paid to do research, teach, and win grants. In their spare time (hah!) they probably would rather go fishing, or read a novel, or spend time with their families than update websites.
The real question is: Why do professors have a website in the first place?
Consider the value proposition -- and then how priorities and technologies change through their career and you get a recipe for out of date sites.
Professors have a website to 1) convey information to students and 2) advertise themselves.
A professor may have a website to post course related files. This used to be the only way to provide supplementary materials and notes to accompany your course. Their university may have adopted some centralized course management software (Blackboard, Moodle, etc.) and now require that such information is shared via that site -- which is only accessible to students. Consequently, the old course pages are left to wither.
A new professor has many reasons to advertise themselves and their lab. They want to attract good grad students and provide information about their work to potential sponsors. They want their papers as accessible as possible (to drive citations). These things (students, funding, papers) are all very important to the professor until they go through the promotion and tenure process. After that, their need to self promote diminishes and you are left with old websites that are left to wither.
In addition to adopting course management systems, departments, colleges, and universities are fond of introducing and changing policies. This often follows a familiar pattern.
Most academics have more things to complete than they have time to complete them.
Some of this work is stuff they're immensely interested in doing (e.g. carrying out work on their key research topics); some of it is stuff they're interested in (e.g. keeping up with the latest research in their and related fields); some of it is stuff that is absolutely mandatory and has hard deadlines (e.g. teaching, marking, some admin); some of is required to keep their work going and has hard deadlines (e.g. applying for grants); some of it is important to keep their work going to a good standard (e.g. preparing and updating course slides); and then you get stuff which would be nice to do and has no hard deadlines (e.g. updating a website).
Which things do you think are most likely to get put off?
The question should be
Why are so many
professors'websites out of date?
and the answer, somewhat tongue-in-cheek is: Because nobody has updated them. The reason for that is, generally, that people do what is fun and rewarding, which mostly excludes updating websites.
As others have already said, this is not something that is entirely under our control. Depending on the institution, there may - or may not - be a person who is specifically tasked with keeping these pages/sites current. Otherwise, it can easily be more than a few years between updates. We, for example, have seen recent staff turnover in our department since the pandemic, and we - only just this week - have identified the person to whom we must now submit our updated CVs. That said, many of us keep our information up-to-date via other means - i.e., Google Scholar, LinkedIn, ResearchGate, etc.
Given most of the previous "complications" and issues already considered, many scholars tend to use nowdays other "technologies" instead, that are much easier to use. Among them I can see (my case) active presence in "social networks" such as Twitter / LinkedIN mostly. Once you register, it is much easier to publish anything related to your work there, and secondly, the platform's algorithm will take care that you get content you like so that you remain there.
Many good answers, one thing I would add is just the idea of pasr historical biases. People who have taught since 1996 are less likely to have their own GitHub. Compare them to young aspiring people in econometrics (like me, or those in software engineering), we (the newer blood) have much much more incentive to want to learn how to make our own website, and maintain it.
Back in ye olden days, you either didn't have a website (since presumably it was rare to have your own) or yours was maintained by those at the university, which as others have mentioned comes with bureaucracy.
