Because the amount of dopamine we get around us is constant, so now people need things condensed to 15 seconds because of the amount of ADHD that's been caused by life having access to the net and immediate hits of dopamine.
Also it's not far off from other shit. Before that, there were Vines. People are just wanting those moment. But I agree with you, I fucking hate Tik Tok. I don't just dislike it, I hate it.