No one is being "forced" to do stuff, it's just that there's a common technical standard called ActivityPub which is used by Mastodon, PeerTube, Pixelfed etc. Because they use the same standard, servers running Activitypub-compatible software are able to communicate with each other.
It's up to each platform's software to decide how it renders content that uses ActivityPub, whether it's from its own platform or elsewhere.