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.
