If I'm not missing something, not all users would know to press the Ctrl+Alt+Del key when working with UAC prompts, and having to press it all the time to verify if it is a legitimate password prompt would be infeasible.
My point isn't that this is convenient for the average user, only that it's something that actually works and is secure. On macs the biometric option might be "secure", but there's nothing preventing a downgrade attack back to password entry.
For the average user security is ensured by not requiring them to enter a password at all, and making it impossible to use a password to get admin access. By default the user is only asked to click yes/no to approve the action. The approval itself is done by the operating system and can't be spoofed. Moreover, the operating system is designed in such a way where even if you somehow were able to phish the user password, it can't be used to get admin access. There's no "sudo" command that you can pipe a password into and get a root shell, for instance.
