I suspect one reason is that they like one-person teams so much. I like one-person projects too! But there is always pain when you have to hand such a project to a new person, because the old one quit, or died, or whatever.
Overzealous secrecy even when not warranted for any actual business-relevant reason also probably inhibits software quality. As does change for change's sake.
[1] EDIT: except maybe Mac OS X 10.6? Does anybody remember any critical bugs in that one? I think that might be the unicorn.