There's a simpler answer: it's hard to make money on open source software. Relatively few projects can generate additional services sufficient to pay even a single author's salary.

Anyway I think the original question kind of misses the point of open source. The big driver for the growth of open source is that it enables users license-free access to large quantities of software. The folks that have made the most money off open source are companies like Facebook/Google that run their business on open source or use it to drive adoption of products they can monetize in other ways.