Fighting on the ground is A LOT messier than this. also technically speaking, kicking up from lying on your back does not generate the same force as a stomp.
If you're on the ground, you're in a good defensive position from which to kick upward at an attacker. You have the ground at your back, which is from a horizontal perspective a solid wall. A kick upward from the ground has the same powerful force as a stomp. Depending on where your attacker is standing in relation to you, you might piston-kick up into his groin, kick out at his knee, or go for a sweep.