What do you mean by cool - cool to touch or not registering on the temperature gauge, there's quite a difference. Cool to touch means around 40 deg, whilst the temperature gauge probably only starts to show at about 90 deg or more.
After 10 miles the engine and gearbox have probably reached full operating temperature - the engine normally takes 2 or 3 miles, the gearbox a bit longer. The engine compartment on most modern cars is well insulated due to it being soundproofed so, when you turn off the engine there is a large mass of metal to cool sitting in a fairly well insulated box. With no movement of the car there is no air flow so cooling will take a fair while as there's nowhere for the heat to go.