@Tim, because most, if not all, GT cars have flat bottoms and diffusers fitted so that it will make more downforce the lower the car is to the track surface up to a point, you would then use packers\bump stops to keep the car from bottoming out on bumpier bits which would remove the ground effect, can make the car real nervous if you over do it because you effectively remove the suspension's travel distance so much that the car cannot transfer it's weight front/back the amount it needs to when under acceleration/braking, if you are a kerb rider it can be lethal for the same reason, classic example is setting car up for the Mulsannes with minimum wing/splitter, minimum ride height and packed up to the max so it don't lose speed scraping it's butt over the bumps then having to baby it through the bendy bits, everything has to be a compromise.
Sam