How many well-formed passwords can and will you remember? How often are you willing to change them afterwards? How secure is your 2FA app? How secure is your device against keylogging? How sure are you no one will watch what you've typed in or when you reveal it to fix a typing error? And so on. It isn't as if there are no issues with the classic way.

That's why you maintain the knowledge-possession separation at the access to your password manager (file) with a combination of a password and either a key file or hardware key.

If you store your secrets in encrypted files rather than specialized server solutions, it's also easier to separate and store them in different locations.