I think I'd argue that game engines are implementing and running multi-threaded engines a heck of a lot these days - to the extent that if a game _is_ heavily using a single core, you'll see complaints.
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)
The latter, from what I understand most engines are multi threaded only in the sense that multiple threads are running stuff, one thread for sound, one thread for simulation etc, and that the graphics processing (still the bottleneck in most games) is still mostly single threaded, at least until it get's to the GPU.
