Because a lot of things still aren't or can't be multi threaded, most notably game engines.
I would be exceptionally surprised to see any of the higher-budget games running on a single thread. Low-budget indie games less so.
Personally, I've found that a good benchmark for "what is happening in most games" is the lowest common denominator - consoles.
Having said this, you might be arguing that it's hard to split everything out equally in a game - but it's far more likely that threads are created for the stuff that can be - physics calculations, sound, etc. (the stuff that might have, in the past, be split out to dedicated hardware)