This makes me question whether you've tried to use TikTok for an extended period of time - say, 30 minutes a day for a month or so.
I liken it more to a skinner box. It will constantly show you low dopamine videos (+ ads) in between "hits" of content that are actually relevant to your interests to keep you hooked. You keep scrolling, scrolling, scrolling past what is basically garbage to find the next "good" video. Sometimes you get many "good" videos in a row!
As far as not being challenged, I'm not sure about that. TikTok is always trying to learn more about you and your interests and hobbies and what videos keep you watching for longer. This means that it also frequently shows you videos outside of your "bubble" as a test to see if you're also interested in other topics. Over the past ~3 years, I have had a ton of engaging conversations with others and discovered SO MANY books, games, TV shows, movies, and hobbies because of what is basically an "everything recommendation engine". Most of the books I read this year (well over 100) were recommended by people on TikTok and were novels that I otherwise would have never even given a second glance.
I have very mixed feelings about TikTok. On one hand, it has led me to so many things I wouldn't have found otherwise (in a way that Reddit, HN, Bluesky, and other communities have failed). But it is also a depressing time suck that can get you to waste hours of your time on garbage and nonsense. Like most things in life, you get out what you put in.