Well you've got to remember that for every potential rebound there are 9 other guys trying to get it too.
I did a little math to show how tough it is. In ACC games last season there were 10,889 FGA and 4,669 FGA, meaning there were 6,220 missed shots or potential rebounds. Across 96 league games, that's an average of 64.8 potential rebounds per game. A player playing 30 minutes a game, had an average of 48.6 potential rebounds he could get. To average 8 boards a game, that player would have to rebound about 1 of every 6 misses, offensive and defensive.
That puts great rebounding into context.