Because in the garage under my jeep, my wife isn't wrapped around me like a heat lamp trying to keep that last hot-dog alive on the rollers at 7-11. And because there's just something about that cool concrete. If I had a comfortable creeper, I'd need an alarm clock out there.
I can relate, my wife and my sleeping preferences are diametrically opposed. I like a nice cool beg, and sleep like a rock if I can stay cool; my wife wants the bed warm enough to boil water.
I almost fell asleep while working on one of the quads. Laying there on the concrete, with just a packing blanket under me, watching the oil go into the bucket. When my head hit the concrete, I woke up...
On board subs, a lot of people bitch about sleeping in the torpedo room (temporary berthing when the regular racks are taken by people more important than you). Surrounded by all of that cool steel, I slept like a rock. Even better when bookended by a couple of torpedoes, those are nice and cool too, and blocks a lot noise.