Why is it that when the car is in gear the tire spins nonstop?

Of course my first question is, are you moving? If not, have you snuck up on it late at night? Is the engine running? It's very common for a tire to spin when the transmission is engaged and engine running, the main purpose of a transmission is to transmit the rotational power of the engine to the tires. You neglected to say whether the tires are in ground contact or not. If they are your idle is set WAY to high, if not, wheel spin is completely normal with either a manual or automatic.