It is this and also that young people are especially malleable, so content that they see can cause long-term psychological and emotional damage, or--more pertinently to governments' interests--can introduce ideological change that they don't control.
1) TikTok is really good at what they do.
2) They are a Chinese-owned company.
It certainly feels like if it weren't for #2, they would be praised for their innovation and held up as a great American success story. I'm not saying that there shouldn't be skepticism in the name of national security, but of all the things threatening to destroy the planet on any given day, this feels low on the list.
YouTube videos are frequently 30-120 seconds of relevant content padded to 10-15 minutes. Most TikTok videos are close to just those 30-120 relevant seconds.
If Congress bans it, they'll probably lose the trust and interest of a massive percentage of Americans under the age of 30.
TikTok has expanded to be the dominant short form video platform for every interest, niche, and micro niche you can imagine that exists, and even ones you can't imagine.
Please anyone reading this comment replace "TikTok = people dancing" with "TikTok = feed + discovery for short form video tailored to your specific interests no matter how niche". Yes this includes programming, science, education, dancing, gaming, cooking, acrobatics, painting, arguing, politics, news, weather, etc. basically anything you can imagine that isn't against their content policies.