Because IRCs aren't social media platforms? Creating an "account" on an IRC server is trivial, and you don't have a permanent profile or friend lists, etc
Speak for yourself, but filling out a web-based form that asks for your desired username, password, and e-mail address is much easier than crafting a special registration request private message to an IRC services bot.
You solve the problem of multiple servers on Mastodon the same way you do on IRC - with a Mastodon client. If anything, it's _much_ easier to keep track of multiple accounts on Mastodon than it is on IRC.
How is this any different than users of different IRC networks being unable to communicate with each other?
If I'm on OFTC, I certainly don't complain about not being able to talk to people on Libera.chat. I just....join the server and start talking.