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.
Yeah, Boris Said is well known in NASCAR not for his prowess in few oval races he’s run but for being the “ringer” on road courses.
These days drivers come from a wide variety of racing disciplines and settle in NASCAR simply because that’s where the money is.