Because shipping charges can go fuck themselves sideways. I still prefer on site purchases over internet shopping when applicable. I see my product as I buy it, I deal with actual people, and if something goes wrong, it's infintely harder to hang up on me in person when I show up and complain.
For something this large in price, I am much more interested in aquiring it in person and dealing with people instead of automated services and programs.
Edit: As for the gamestop thing, powerup rewards is pretty much the only reason I bother with them still. I got 8000 points accrued, and after the ps4 purchase it will probably jump to more like 15k.