The other thing that slows them down is the need for a meaty human in the cockpit. A fan car (creating downforce by pulling the car to the ground) can go faster than a human driver could reasonably sustain without passing out in the corners
No wonder F1 drivers have necks as big as their head.
> There was fear of the possibility that drivers could suffer "grey-outs" or lose consciousness from g-LOC. It is also likely that the high g-loads would have been outside the design limits for the HANS device, which was required for all CART races at oval tracks.