Driving in a stock car doesn't seem athletic, but there's a few factors that make it a pretty demanding event.
Mainly, they're strapped very tightly in a car in an already warm environment. Temperatures inside cars get up to 140 F quickly, and they have to deal with that for upwards of three hours. This produces a level of sweat and tiring comparable to a strenous football game.
Also, the speed at which the cars travel often puts more than three G's of force on the driver, more than any standard sport.
It's not a sport in the same way team sports involving passing an object, defensive maneuvers, and role players are, but it's still a sport.