That's a pretty good question.
I'd say the average for people who have been playing consistently for at least a few years and have a lasting interest in improving would be something like 1600-1700.
Of course there are many people who play 10+ years with little to no interest in improving who never get to 1600.
Is it because most people play for a while and then quit? If so, is there a way to see the average rating of chess players who've been playing for more than 2 or 3 years?
Or is chess just hard and an Elo of 800 really IS the average rating of a casual chess player?