[traviswingo]

IMHO it's because, once you decide to go open source, you're creating with the intention that this is a "for the community" type of product, not a "for profit." It's like a non-profit company with unpaid volunteers. There's a cause that many people believe in and anyone can help work towards that cause to make it a reality.

[AstralStorm]

Moreover, many critical projects are already done by FSF or Apache Foundation, which are non-profit. Other by RedHat, IBM, Google or Oracle, which already monetize them.

Some were done in a "scratch an itch" fashion and author is not interested in maintenance.

Some are research work where a given university (and not the author) would have to draft a contract for anything monetary.