In a nutshell, it's because you don't have to build the entire kit and kaboodle all at once.

Lots of BlueSky is centralized so you don't have to worry about distribution, user, hosting, scaling, etc. and just focus on the frontend.

It's the same reason why all the Lemmy and Mastodon apps look way better than the web versions, because all those other parts are no longer relevant and the creator(s) can focus on just putting out a polished product.