TikTok isn't about movies, it's about short videos pulling in your attention. They aim for the mobile phone.

Also if a video is showing a single person (standup comedy?) vertical can show them closer, as people (mostly) are vertical.

But yes, for a full movie, where you have horizontal camera movement, a vertical screen gives too little orientation for following the movement.

In painting and photography horizontal and vertical coexist for millenia. Based on the motive and purpose.