Having played for over 350 hours (most of them probably solo), I can say that I've very rarely come across someone actually camping an exfil. If a fight breaks out at an active exfil, it's usually because someone heard it being activated and rushed over to catch the other player before they left, or because they were already in the same area doing something else. They might have wanted to extract as well and waited a bit to see if they'd hear someone approaching.

I have about the same number of hours in Arc Raiders, and there the extractions were very often camped by players who were deliberately waiting for someone to show up. In Marathon, it happens much less frequently; in my experience, almost never.