Because the Pros use the same screen technology that has been used since... 2008. It's costly to rework the fabs for a different kind of screen.
On the other hand, the Air uses a completely different screen that's thinner, and based on newer technology so it can afford more pixels. What it can't afford is better color quality, and when Apple decides to revamp the Pro line, no doubt the 13" would have the same resolution but with better colors. Or we might even see a 2560 x 1600 screen for the 13" for HiDPI. In such a case, the 1280 x 800 resolution on the 13" right now makes perfect sense (when they double it).