NASCAR Racing: Cooling Masking tape, nascar cars, brake bias

Question
I am curious why NASCAR cars block off areas of the grill with tape (and later remove it)to control cooling.
Since this seems to be a constant issue to maintain proper cooling, wouldn't it be more efficient to control with louvers manualy or thermostatically. Tape seem to be such an arcaic method.

I know that the only driver adjustment in the car is the brake bias, but even outside control in the pit would seem to be a better method then tape.  Any rule insight?

Answer
I suppose the answer is held in the phrase "keep it simple". Considering teams only have pit stops to make adjustments, and a limited number of drivers allowed over the wall, tape is the most reasonable method to control cooling. I don't believe there is any specific rule stating it's the only way to do it, but it seems to be the only way teams can really make it work. It works well enough, unless something gets caught on the grill and causes it to over heat.