It isn't even only that.

It's people being told that the Fediverse equals Mastodon for simplicity reasons. Because that's easier to understand than the Fediverse being a network of over 100 different apps with hundreds or thousands of servers each.

And it's people being told that the Fediverse equals Mastodon because whoever tells them that believes it themself.

Most Mastodon onboarding is done by people who think that the Fediverse is only Mastodon. At least half of them will spontaneously shit brix once they come across a message from e.g. Friendica or Hubzilla for the first time.