Turns out that there's a lot of access to Microsoft's codebase as well, it just happens differently.
There's sharing with various vendors, including device driver and applications development companies, to greater or lesser extents.
More significantly, there was a program under which universities (notably in Canada as I recall) had access to much or all of the NT / XP code base under a restricted license. The possible scope of exposure was huge.
And of course, there are unintentional disclosures -- I've heard of, and been at, companies whose source code was leaked, often in its totality, sometimes multiple times.
The difference being that those who receive such code cannot comment openly on it or provide fixes to bugs as is the case in Free Software. They're limited to identifying exploits for their own benefit or others'.
I don't understand why not most old software (OSes, programs, games) aren't commonly made open source though, it's not as if anyone is making money with Windows 95 these days.
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.
This article here explains this: http://sealedabstract.com/rants/beyond-open-source/