Users reuse passwords. We've had decades of trying to educate people to do something else (use a password manager! come up with a unique password and keep it in a little book! anything!) but it simply hasn't worked.

That's an engineering fact. It would be good if it weren't true, just as it would be good if virtual memory were indistinguishable from RAM, but it just ain't so.

To be a responsible engineer, you've got to design and build for the real world, and that means not relying solely on username and password for extremely sensitive data.