It kind of makes me think of how odd it would have been if many of the old forums named themselves like bookclub.phpbulletin.com, metalheads.vbulletin.net, or something.
There’s nothing wrong with doing that, obviously, but it’s struck me as another interesting quirk of fediverse instances/sites. Generally as soon as you visit them you can tell by the site interface or an icon somewhere what software they’re using.
You also can’t reuse a domain between software installations (some exceptions apply when migrating between software of the same “family tree”, e.g. migrating from a mastodon instance to glitch, or migrating between misskey forks) due to how federation works. Hell, reinstalling the same exact software can break federation if you wiped your database in the meanwhile.
Some software offer a “split domain” option where the software itself is installed in a subdomain like mastodon.example.com but with user handles on a separate domain (usually the root domain, like @example.com) but I am not too sure on the reusability of that, and it’s not an easy thing to implement (Lemmy won’t deal with that correctly and will always use the full domain for anyone on a split domain instance). There are also a handful of software (like Takahe) which let you “bring your own domain” so to speak.