Because my time is worth more to me than that. It's not so much the travel... it's the waiting once you get there for the food to actually be ready (or conversely, waiting at home first and risking it not being hot when you get there).
I prefer to pick up the food myself, so that I can be reasonably assured that it's hot when I sit down to eat it. I've gone so far as to buy an insulated pizza delivery bag so we can eat hot pizza at home.
For me, the prospect of lukewarm food kills the delivery experience for me, even if delivery were "free" (not itemized to me).
