It's not that technology itself is bad for children, but, if not content-regulated, it can serve inappropriate content (think 4chan) and, if not time-regulated, it can take time away from learning other important skills. Technology can also serve up stuff that's...not so useful to a child's development, if not necessarily completely unsuitable (think video-game Let's Play videos).
Technology as a tool is great but all tools have their limitations. It can maybe help a child learn words or help a child's understanding of a skill, but it can't teach a child to interact with others, share toys, let a child actually practice said skills (outside of technology-related ones like coding), it can't get them to exercise, it can't provide interactivity beyond scripted events and prepared responses to touches/clicks, I could go on.
Content available on said technology is also very good at being flashy and drawing people in. Some of the best video games have what we call a 'one-more-go' factor, some of the best shows are good at delivering cliffhanger endings to conclude episodes, and a lot of content is designed around making the consumer want more. And kids are generally not very good at time management or prioritisation on their own, so time management needs to be enforced by the parent.
Finally, technology almost always puts the user in the driver seat regarding what gets done. It usually doesn't care if said user is a 4 year old; it will serve the same internet an 18 year old will see and the same content an 18 year old will have access to. There are some ways to enforce technical restrictions but no such system can be perfect and there will always be holes. There is no substitute for parental supervision.