Free throws, foul calls, any violation basically and the clock and play is stopped. Basketball has 4 12 minute quarters which would mean it's a 48 minute game but it isn't. It usually averages to 2 hrs and 15 minutes or so. What's that, 135 minutes? So subtracting down the 15 minute half time plus game time of 48 minutes, there's 72 minutes of basketball not being played in a normal game. I don't think it's as constant as you think it is.
Have you ever played full court basketball? It's incredibly exhausting ... people seem to forget just how easy the pros make their respective sports look ...