It's a bunch of work, often involving lawyers who bill by the very expensive hour, for basically no payoff. On top of that there's a secret in the industry though: there's a lot of maybe not entirely legal code copying going on. Hiring someone who's made a similar game in the past can mean that they come with source code that could be copied to shortcut the time to write code for the new game. Releasing source would…
Even if they're in-house corporate lawyers, it's still a lot of work that is a distraction from their day jobs. And, yes, there may easily be some combination of proprietary libraries and other code that isn't entirely kosher to open source in some way. And, oh, you want the build system for that code? That's something else. In a lot of cases it would involve tossing an incomplete bunch of code over the wall with few…
The "recipients" don't mind this at all. Many source ports built from gutted releases speak to that.