So, firstly, Postgres takes responsibility for contributions. The fact that someone has submitted a patch or PR is no guarantee that it has been accepted.
Secondly, they combine all those contributors to release specific versions with specific features. I can trust that someone at postgres has reviewed all those contributions to ensure they make the grade and included them in the release for a reason.
Thirdly, Postgres as an institution takes responsibility for the code it releases. If I find something messed up in a postgres release, and report it to postgres, they will take responsibility for that and manage the rest of it.
None of this is true for NPM.