Because a potential intruder is more probable to try "root" than "akerl_35" to get access. It's no big deal to login as any other user and then use sudo (if needed) or even su
If a potential intruder knowing what username to use helps them bruteforce your SSH, then the problem is with the entropy of your password, not your choice of username.
Yes, but it's easier to teach admins to never use "PermitRootLogin yes" "because it's bad for security" than to teach them to never use weak passwords.
