I think its just a numbers thing and what is easier to understand or keep track of game by game.Its similar to hits in baseball, which we use, but, the batting average is really what's used far more regularly, pretty much the standard.
For example, in hoops its easier to say Player X averages 22.7 ppg rather than saying Player X has 1578 pts this season. Just like in baseball its easier to say Player X is hitting .317 rather than saying Player X has 153 hits this season. Additionally, in baseball one guy might have more hits because he has 50 (or whatever number) more at bats than another guy who actually has a higher average. As far as HRs and RBI in baseball it would be a lot tougher to see the difference between guys if it was based on per game averages... Example: HRs per game... C. Davis 0.4, Puig 0.3, Cabrera 0.3, Dunn 0.3, Dom. Brown 0.3 --- RBI per game... Cabrera 1.0, C. Davis, 1.0, Ortiz 0.9, Goldschmidt 0.9, Cuddyer 0.8
Make sense?