Road racers do pretty well on the NASCAR road courses but these days many regular NASCAR drivers have considerable experience on road courses.
Only because they test and train alot for it. You don’t have to remember the days when the “ringers” are brought in for the road races. As a turn about, A.J. Almendinger, Mike McDowell, JPM (Montoya), Andy Lallly to mention the current crossovers from road racing to corcle track require virtually the same amount of experience.
My point is that these guys are very good at what they do and to take a relatively same set of skills to do both, it is not easy to hone those talents to be just as good at both. That is just how demanding the athleticism of the sport is.