A lot of companies are just fixing stuff for their software and hardware, by submitting patches it almost guarantees it will turn up everywhere - I could imagine getting Microsoft or Apple to provide a feature to all their customers is a lot harder.
The other reason is maintainability, it's time consuming to maintain a patch outside the tree and people will end up submitting them so that it doesn't get bitrot.
Also consider this with regard to other open source projects:
Proprietary PostgreSQL vendors often contribute a heck of a lot of source code back for a reason that seems obvious when you think about it and applies to something like Linux too. The basic issue is that if you keep your own private fork and maximize what's not shared then you also maximize the work you have integrating what everyone else is contributing. By giving back as much as you can, you cut your costs down the road. That isn't to say that EnterpriseDB gives back everything but they back quite a lot.
When it comes to a proprietary vendor like Microsoft, it is much harder to get the QA done, and get updated drivers out to customers.
Having dealt with Microsoft support, you are right. They genuinely don't want to provide a code fix as that means hassle. You just don't get that with open source whether you fix it yourself or report a bug.
The other reason is maintainability, it's time consuming to maintain a patch outside the tree and people will end up submitting them so that it doesn't get bitrot.
Doesn't really seem altruistic to me.