I guess part of it is that each platform wants to be specific to the purpose of its own network/platform. Mastodon as a Twitter/Tumblr like microblog site, PixelFed like Instagram, Friendica like Facebook, so on and so forth. Align all terminology to one set of terms I think takes away from the uniqueness of each platform. If you want the truly neutral default cross-Fediverse term, then just use the one from the ActivityPub spec. That pleases every Fediverse platform equally, i.e. not at all.
When one network interacts on another, the target will do its best to translate your actions into the way it does things. That’s why Link posts from Mastodon appear so wonky in Lemmy, and you can only reply to Mastodon users who posted in a Lemmy community.