Despite what it might seem like from HN users, the vast majority of people don't use password managers.
If there was a well written protocol and library to help with the independent federation of identities, that could be used by anyone, that would truly help the security of the vast majority of people.
So it's only a massive target if you then recentralize those credential databases on a cloud storage service, for example.
Attacking the security of a password management system is different than stealing the credential database.
If it's a federated system, there's already more than one provider. If it's a "strong" provider, then that probably means it's a market leader, in which case there's an equally strong incentive to edge the competition out.