Probably because some people tend to pick user names that identify them in some way. Take me for example, I have a few names I go by but this username is definitely helpful in identifying me. I use it on the other place, a couple of emails, discord, telegram, etc. I don't feel the need to be as anon as possible (no shade on those who do) so I main this one. I have a few others that I have been known to use and those are mainly for things that I don't want easily connected back to me.
You shouldn't be forced to be anonymous. If you want to pick the same username, you should be able to. But even so, there's still a required number at the end. So unless your username elsewhere ends in 2 digits and isn't already taken, then you can't pick it anyway
It's not about forcing anyone to be anonymous. I'm not OP here but I kind of agree. Maybe signal should default to a randomized one with a blurb about safety, anonymity, etc but let you create your own if you want.
Again. My personal view isn't to force random usernames on people but to maybe educate them on this stuff. Also, there are legit reasons why you should have non identifying usernames even if it's not how the world should work. There are enough nutters out there who may recognize something in someones name that links them to someone they know offline and people are nucking futz. I can tell you stories I've heard from my clients that you would believe but don't want to.
Oh and for the numbers, that can be even more identifying because people tend to use numbers that mean something to them. I have a variation on this name that includes my birth year in 2 digits. If I was posting things online that close family might have a problem with, it wouldn't be hard to do to the math and identify me that way.
guessing it would mean that people wont be using the same username as they do on every other account. So if doodlebop69 can't be traced from signal they could go to google and find the same doodlebop69 to grab their information from