But open-source doesn't always mean working for free, nor does it mean people do it for purely ethical (or socialist?) reason.

There are lots of reason why open-source is attractive after discounting ethics and money. I imagine being credited for being a major contributor to a popular open-source project would mean better job opportunity in the competitive tech job market. The gig doesn't directly offer you money, but it does gravitate the right company that has the money to fund your work they find very valuable. In a sense, this isn't that far from how capitalism work -- credits are due to the people who brings most value to the society, whether the source of the software are open to all or not.

This is of course a very superficial statement to make, but I remember Eric Raymond wrote about this in more a detailed (and more convincing!) manner in The Cathedral and the Bazaar.