I'm not sure I understand the "open source" part - it's not as if we do it for free. I'd say for most of the senior PostgreSQL engineers / contributors it's part of their paid jobs. We either work for companies providing various PostgreSQL services, or for companies using PostgreSQL.

We might do the same type of work on a proprietary database, but that would lack the community/cooperation aspect of the project.

Of course, enjoying the craft is somewhat natural in both cases. And yes, it's a bit scary how smart some of the contributors are.