Why our username can’t be used to be identified ?
NIP-05 identity. It allows you to easily be identified by something that looks like an email address, instead of a very long and complicated public key. It helps you be found and searched for too. You can read more about it here. https://github.com/nostr-protocol/nips/blob/master/05.md
You can host this yourself if you own a domain name and have a web server. It's honestly a very simple task, but not everyone has these things. So, I setup nostrplebs.com to help users out. You can pay a small fee in sats and get setup in no time. Other providers now exists too.
Discussion
Because that's not unique. You can have 10,000 @billy's out there all having the same username. Only 1 @billy@domain1.com can exist. And, @billy@domain2.com can exist and so forth. Billy Bob could tell everyone that he's the @billy@domain1.com and not any other Billy.
