It's not the same issue. With your approach, a compromise of the seed is catastrophic and reveals everything. With a regular password manager, you also need access to the vault encrypted using that master password. That's not a given -- most password managers either store their vaults locally or offer 2FA when synced.

(Nevermind that you can't change individual passwords or the master password at will with a deterministic scheme.)

and the virtue of allowing a single point of 2FA.

the seed is just the seed, and will always be the seed. the master password can change and be supplemented by 2FA / other enhancement schemes.