[3princip]

If your primary motivation is short-term monetary gain then you're right, it doesn't sound like a rational undertaking.

I can think of two reasons to work on open source. Altruism, you want to give back to the community without expecting a monetary gain in return. Investment in skills, if you want to differentiate yourself from peers, you'll have something to talk about to potential employers. It is a great opportunity to learn and become a better software engineer.

[fuzzfactor]

Also, some of the best software could be stuff that nobody can actually afford.