Having a constant wheel size and regulation is one of the things that makes touring a good class with excellent continuity- like 1/8th buggy. With wide tires we'd all need to run new suspension geometry, knuckles, and such. There would be a long period of innovation to find the best solution that is would cost racers an arm and a leg to keep up with. There are lots of things that could make our cars faster, but that doesn't mean we should actually do some of those things.