Most cited reason I've seen is the patents/copyright mess. Just look at how System Shock 2 is handled for example. Patches being anonymously contributed by a group/person that is said to have access to the source.

There's also the situation with the lost source code. For example, as I've understood it, the only reason the Dark Engine source code became available were due to someone finding it in an old Dreamcast SDK.

I gave the same argument, and was met with things like "because unfettered access to old OS source code might reveal new vulnerabilities in our later OS source code." To which I responded "good, then you can fix them and have better source code"