Because TV is mostly boring shit like commercials and repeats. It lets your mind wander. TikTok feeds you content without distraction until eating laundry detergent seems like a great idea.

https://www.youtube.com/watch?v=tdIUMkXxtHg

I remember watching a video about this topic (sources in the description). The tl;dw of it is:

The content isn't the (main) problem. Its the format. The infinite scroll + short form content teaches you to be impatient. You could watch a montage of cat vids that cuts every 10 seconds and be fine. But have a tiktok style feed of the same content; having to swipe to see the next thing. You'll preform worse in cognitive heavy tasks.

Obviously its more nuanced than just this but yea.

I honestly think speed has a lot to do with it. With the rapid switch and ability to just ping the next thing, it removes all reflection time.

Even flipping through channels required enough to evaluate what you were jumping into the middle of.

Algorithms make it so the videos are incentivize to onboard you as fast as possible and they start when you flip.

There's this: https://pmc.ncbi.nlm.nih.gov/articles/PMC10030518/

Pretty sure people who watch too much faux news also experience cognitive decline.