The problem is that would be so annoying/impractical. In an optimal world, yeah a person checking a prompt and approving could make sense, but in practice that would also mean that the MFA prompt would have to ask for the password anyways. (Or the password would be on the phone with the same problem as on the computer).
Can you imagine having to type a password on an hourly schedule or something? If the password was cached, we have the same problem again.