Because somebody had already made the app and we have just integrated it into KDE. Also, it can work as a standalone animation app and is used to animate stuff that have nothing to do with Kdenlive. Why would we want to cut it down to a plugin-like thing of Kdenlive?

Probably because the devs didn't want to. Which is reason enough. My experience tells me the krita crew are more focused on painting and illustration. Could animation be shoehorned in there? Sure. Would it likely be as good of an app for animation? Likely not.

I love blender and use it all the time for modeling etc. You can also NLE in blender. It doesn't currently hold a candle to kden live or davinci in a number of areas for that. But you technically can. Same goes for compositing etc. Again it's not top of the heap for that. They're much more focused on modeling and some on rendering. Which have both made massive strides. Splitting their focus further likely wouldn't have made things better over all. More likely worse.

I'm glad to see this app and will be checking for AUR or flatpak soon as I get home myself.