Your message to me reads like someone who is upset, so right up front I'm going to say that I understand where you're coming from and it's not wrong to feel the way you do about this.
However, the other reply-er is right about the security problems with what you're asking, and I'm going to try to illustrate why.
If that's not something you're interested in, you don't have to read this. I'm not calling you out or anything, I'm just trying to fill you in.
Let's imagine I'm a hacker, right?
And I want to get into a particular service, but I don't have any credentials. How to solve this problem?
Let's just try figuring out the usernames in-use first.
I make a web request to the sign in service. I get a "404 Not Found" response (or, in this case, a "no user exists with that username" message on screen)
Awesome, I'll just write a quick script that hammers the sign-on endpoint with every conceivable username I can enumerate. Most get 404s, a few get "403 Unauthorized" (or an "invalid password" message). Those represent real usernames. Great!
Now I just gotta find one person from this list with a predictable password, or enough of an identifiable username to target with a social engineering attack. Bonus points if they have elevated access.
It's true that 2FA, IP based bans of bad actors, passkeys, strong password requirements, password expiration, and more can make this job a little harder, but there's workarounds for all these problems.
That's what they're saying. Not that the server returns your username, just that by providing different responses to to failed login attempts, you make usernames extractable based on your response behavior.
I hope this helps you understand the reasons for the frustrating user experience. Its because they're trying to protect your data (username) from hackers.