Aside from third party licensing for middleware like Bink, Scaleform, Havok, console APIs, etc. I'd guess some if it has to do with the constant competition over graphics technology that's only just started to slow down in the last couple years, and perhaps that will be picking up again with the next-gen consoles.
I completely agree, though. There is a complete lack of solid open-source game software.
I completely agree, though. There is a complete lack of solid open-source game software.