If the various "redditish" projects could agree an shared framework providing access to all the features common across all implementations then sure, it'd be easy. But the thing is, that things are done just different enough that dedicated apps make sense to make use of any unique features.
Also, I've been using my phone's browser to post to kbin and I think it is fine.
shared framework providing access to all the features common across all implementations
Thats what web browsers are supposed to do.
It is the exact role they serve.
The last 2 gemerations of 'the web' have added little new capability, instead inventing new expensive ways to do what was once free all while invading our privacy.
All an app does is add another layer for someone else's profit.