It's a hard user experience design problem to create an interface that presents all possible types of posts, content and interactions in a sensible way. This "kitchen sink" approach is kind of what Facebook does and as a result its interface is messy and cluttered. That's not to say it's impossible or wrong to do things that way, just difficult and unpopular.

On the technical side, it's really hard to make a client app that works with multiple server softwares, because they all have different sets of features.

In the current world of fedi software development, it would be a single dev or a small, likely unpaid team that would have to make the equivalent of several different client apps combined into one. I don't anticipate such a large and complicated project being completed until the devs can make a decent living doing the work.