We totally agree!
If a user is banned, he/she deserves to know why. You can look up your username in the Mod Log and see the reason and the expiration date, but it is not user friendly. Since we cannot do something about this, it is up to Lemmy itself to fix this. I suggest to do a feature request on Lemmy's GitHub.
Regarding the who in your question. We understand that you know who did ban you, and you get the information if it was a mod or admin. If you're banned you can always appeal! Mail us at info@lemmy.world with your username. We have a system internally that ensures another admin reviews your appeal. That way it stays fair.
The reason we have hidden usernames in the Mod Log is to prevent harassment. This occurred several times when the usernames were not hidden.