Well, actually, heat is trapped between the wheel wells and the hood. The hotter air is radiated or pushes its way up and tumbles. This causes a cushion that prevents air racing past from coming up into the engine bay. While air is always moving and hot air is always escaping, the air temp inside the engine compartment will always be hotter. While I am not arguing how effective a blanket is on retaining heat. It certainly isn't helping.

Also, its has been tested repeatedly that a vented hood reduces heat in the engine compartment. Car manufactures could careless as a vented hood also means reduced aerodynamics and in turn reduced mpg. Performance vehicles will factor intake temps into the equation by moving the intake port near the wheel well or other areas where they can grab cooler air.