Why are Stack Exchange sites too complicated to understand? [closed]

This is... not an easy question to answer. The sites in Stack Exchange network (typically) require some degree of professionalism, yes. That said, a decent amount of expectations from a "be nice" point of view are in the Code of Conduct.

Different sites require different tasks. To provide a detailed answer, to, say, a Java question on Stack Overflow, chances are you'll need some understanding of Java. There's also many sites that don't require a technical background, such as, say, Gardening Stack Exchange. So all sites certainly don't "demand" it, but it can be useful on some of the more technical sites, yes.

In regards to your title, that's somewhat of a challenging question. Stack Exchange (for the most part) is quite different than other sites. We have far more rules, and far faster and stricter moderation compared to, say, Reddit. This isn't true of all sites, but it's certainly true on some of the larger sites, like Stack Overflow. To quote Mithical in this answer

There is a really steep learning curve for the site. Stack Overflow is, objectively, really different from most sites on the internet. We don't take subjective questions. We don't allow open-ended requests. We don't allow polls. We don't even allow posts about computers that aren't directly programming.

If you arrive from Reddit or Quora, you're going to encounter a massive culture shock. It's just so different in what's acceptable here.

So when someone arrives, and they post something that would be acceptable anywhere else on the internet, and within ten minutes they've been downvoted twelve times, had their question closed so that nobody can answer, and had three comments telling them that their question wasn't acceptable... that's frustrating, to say the least.

It takes a bit to learn how to properly ask a question on SO - and to learn when a question would fit better on SU or SF, for example. In the interim, you get slammed with downvotes and closures and comments. It's easy to feel attacked.

Although I realize it isn't exactly what you're asking, I'd actually suggest reading this answer (the rest of what I quoted) fully. No guarantees, of course, but it may be helpful.