Keep in mind that there are tons of gratis, non-commercial indie games, too.
Most of them don't get source code releases because their authors didn't
think of the possibility that it could be important.
I don't think not being able to get rich is the problem with commercial
games, though. It's a well-known fact that video game developers don't make a
whole lot of money; the average salary for a commercial video game developer,
IIRC, is around $30,000-$40,000, a lot less than other programming jobs. They
do it because they want to make games, not because they think they'll get
rich. The only ones who think they can get rich are naive amateurs. This
doesn't say anything about video game publishing corporations; of course
corporations put the bottom line above everything else, but I think many
indie commercial game developers would be fine with releasing their source
code if they knew that they could still make a decent wage. I've written an
article about that actually:
http://www.gamedev.net/page/resources/_/business/business-and-law/you-dont-need-to-hide-your-source-code-r3503
