Some friends and I were discussing why we do free software even though it often means doing tons of work for little or no money.
I think for me some major factors are the personal agency you have over the work (independent from employers/investors) and the potential for longevity that comes with having your work be part of the commons (projects can't be acquired and killed).
That kind of unalienated relationship to your creative output is very rare outside of maybe fine arts or entertainment.