To set a scene, you awake in the middle of the night because your phone is making noise. Blearily you unlock it, glance at a prompt, and then approve a login and fall back asleep. The intruder now has access to your password manager!
They attempt to log into your bank and drain your life savings, but despite having your password it sends another prompt to your phone. This time, you wake up enough to realize something is wrong. This time, you deny the prompt.
The entire second paragraph cannot happen if your MFA is a single factor. Don't store MFA in your password manager!