Usually you have to fight for the right to contribute back, which in big companies means lots of fun with upper management layers and legal department before you get the green light to do that little pull request.
So in my career I have seen lots of open source being used at Fortune 500, sometimes with internal forks of bug fixes and zero contributions being done back.
Yes it is.
But once you start making serious changes to the source-code, it's probably less painful than trying to deal with all the merge conflicts from your own fixes every time you upgrade to a new version. At some point it's just easier to push all of that stuff to the common version.
What matters are all those companies that no one heard of, or whose business has nothing to do with producing software.