Hard coding other website engine names into the structure of your own website engine seems... not great.
Lemmy doesn't show posts from Mastodon for the exact same reason it doesn't show posts from Loops. Or PixelFed. Or Miskey. Or Bookwyrm. Or other Lemmy users.
A "Mastodon" feed would encompass all of those things. And it would promote the idea that all that exists outside of Lemmy are the most popular wish.com versions of existing mainstream services. It neither leaves room for acknowledging less popular options, nor anything that's genuinely new.
Plus, there are already website engines that let you follow both groups and users. If that's something you want, mbin is right there. Why not use that?
Well, no point in resisting America's bullshit if it involves changing daily routines, I guess.
Jesus, no wonder we keep failing to change literally anything.