Do you want a suitably silly answer for your silly question?

Here are three good reasons to use a plenum; aerodynamics, filtration, and heat.

Drag cars don't have to worry about aerodynamics during cornering, and having a velocity stack-style intake means having a scoop of some type on the hood, or just letting them hang in the breeze through a hole in the hood, neither of which is a particularly efficient way of keeping air flowing smoothly over the top of the car. You can push the proverbial barn door through the air pretty quickly with enough brute power over a short course, but road racing is a game of seconds gained here and there, and barn doors perform quite as well with frequent power transitions.

Drag racing also doesn't really involve the concept of "endurance" to the same degree as road racing, since you're only running an engine in anger for maybe 10-12 seconds at a time, tops. A 24-hour endurance race like Le Mans demands an excellent filtration system, especially because of all of the rubber being thrown from the tires of other cars. Ever see pictures of the C5-Rs after a race? They start out yellow (and white) and finish mostly sooty black.

And finally, pulling air from the front of the car for the engine is obviously to be preferred to pulling super heated air from under the hood. Cooler air means more power. A plenum allows you to ram air from the front of the car directly to the intake runners.

 

"Ever see pictures of the C5-Rs after a race? They start out yellow (and white) and finish mostly sooty black."

Yeah and this year it will be because of the Audi Diesels.

Bret