Why can’t every webpage show the password requirements if entered wrong?

Security reasons, the more ambiguous you can keep it, the better it is for keeping bad actors away.

Same reason almost all sites don't tell you whether it's the email or password that was wrong when you create an invalid login attempt.

That would be helping the hackers, by saving them the 10 ~ 90 seconds at the start of their session it would take to determine the password requirements manually.