Because they want to force the content on you. It's not the goal that you can choose to enjoy TikTok-like short videos. You can already do that. They want to mix the videos into your feed so that you get caught in the net, like they already do on their app.

It’s likely that very few Reddit actually users want short form video. And that the acquisition was mostly for the user base which they then tried desperately to hold on to.

So the obvious answer is to create a standalone service… which it was before they acquired it.

I don’t understand why it has to be an app at all. Why do they want people in an app rather than in a browser?

Because you can get more data to sell through an app.

And it's harder to block ads/sponsored content. And they can send annoying push notifications that lots of people don't know how to disable. Dark patterns all the way down.

I tried to official app recently, and while I did figure out how to remove ads using ReVanced, I don't think I managed to get the notifications I wanted (replies) without notifications I didn't want.