Because I already use MFA to access my password manager in the first place, and don't want to deal with managing backups for each flavor of MFA app that is pushed on me.
How do you manage MFA for encryption-at-rest? None of the common TOTP systems do this. LastPass and 1Pass have built-in "local encryption keys", but they're stored in the same place as the store and only protected by your password. I think theoretically you could set this up with Keepass using a Composite Master Key (combining a password-protected key and a certificate-protected key, storing the certificate separately, ideally in an HKM), but I don't know anyone who does this.