because that is your identity, the full "user@domain" the domain you are on is an intrinsic part of addressing to get to your account. so the question then becomes why can't you change your name? and it has mainly to do with avoiding name conflicts in administratively isolated systems.
A major problem in distributed systems is names. one solution is to do what dns did, you can have nice names but you only get to pick them under the part of the hierarchy you control. another solution is to do what git or ipfs does. nobody gets useful names but at least you don't need any sort of central name database.
mastodon went the dns/email route, which makes sense people want nice names. but now your name is stuck on your domain. perhaps someone could have setup a central name server to avoid name collisions, but who would you trust to run it? what happens when it goes down? might as well just use dns.
Off tangent opinion on names in a federated system.
Unfortunately mastodon adopted the twitter style "@user" but because this only make sense in the context of a single domain mastodon mainly uses the awkward form "@user@domain" I think the email/xmpp form "user@domain" would have been better, but if they felt the @ prefix was critical to the experience of a twitter like micro blog than they probably should have adopted the form "@user.domain"