How common is it for studios to outsource audio work to freelancers or post-production studios?

I've been looking at dozens of "careers" pages on different studios' websites (AAA, small, and everything in between). Many seem to have one audio opening available, fewer still having more than one opening. However, a large number of these studios don't have any postings let alone job categories for audio/sound departments.

As someone coming from audio post-production in the video production industry, I wonder if part of the reason why many of these studios don't even have an audio category in their job listings is that they're outsourcing their audio work to freelancers or post houses? And further to that point, are there game-audio-specific post-production houses out there?

How common is it for game studios to have no audio department at all, or not even a single audio-specialized employee?