While it's true that helical can transmit more power, because the force is spread out more, spur gears usually have stronger individual teeth, and they transmit force more directly. You can think of them as more efficient than helical gears. It's the stronger teeth that racers want, I believe. With helical gears, there are more losses, I believe, and racers don't want that.
One reason spur gears are used in low speed apps is because when you move them fast, they mesh with a whine. Which is what you are hearing in those GT cars...Spur gears meshing fast and noisily.
I believe you can make spur gears quiet by manipulating the mesh angles, but it loses something, making it undesirable for racers.