Because it's reasonable to assume that both those guys will play a full 82 games this season, or going forward? I'm not skewing stats. I'm going by their totals. Neither of those two will ever play a full 82 games again. So why would you go by their full season projections?
Because a per game estimate is a far better predictor of what they'll do in a single game than their recent season totals would suggest. You'd be correct if they were playing the emtire season together, but they're not.