Why does the engine rev up when you start the car?

This is because the engine is cold. The oil is all at the bottom, and does not want to move around. So the engine revs up more because it is trying to produce heat and get the oil circulating as fast as possible. The engine runs in an open loop, meaning the computer has a preset running condition that is in effect until the car warms up enough for the computer to monitor the engine accurately for itself.