It's just difficult to add the proper amount of "bumpyness" if everything's fictional. You either end up super smooth (a la LFS) or you've introduced something that's ridiculous and equally fake by being too bumpy
When driving a laser scanned real track, it's not so much that I care about the exact bumps for my own driving training, it's that bumps actually form due to some natural process, be it that it's the curb lane so larger trucks have caused the lane to sink a bit, or just that the track is in a rainy climate so the track erodes or causes the soil to shift under the weight of the track surface.