I did a quick averge of the % of points that a team's leading scorer gets. I took seven teams, and the average comes out to 21%. At that rate, for a player to average 20 ppg, his team has to average 95.2 ppg, which is WAY above average. The highest % of the seven teams was 27.6, in which scenario the team would have to average 72 ppg.
I don't know what seven teams you utilized for this study, but it does give some perspective. When I think of "averages", I think of the old story that if someone has one foot in the fire and one on a block of ice, on the average he'd be comfortable.
My point was that in order to have a player score 20 ppg, you either have to have a very high-scoring team or have one player that takes an unusually large percentage of your shots - or shoots awfully well. Even good shooting won't get 20 ppg on most teams.