That's because Mastodon, PixelFed, Lemmy etc etc are different services based on ActivityPub, they aren't instances of the same thing; what you want is a software that can communicate with Mastodon, PixelFed and Lemmy

TL;DR: the softwares themselves have to be made in a way to be compatible with eachothers, eg: Lemmy and PieFed works with eachother or you can also use Mastodon to interact with Lemmy (but not viceversa IIRC) too