Why do they call them "stock cars"?

Check out the Speed Channel. It covers quite a few racing series that feature production-based cars, such as the following (in increasing levels of sophistication and speed):


(There’s also the Spec Miata Race Series, that is open to the older Miatas, but I don’t think they get on TV very often.)
The SCCA Speed World Challenge features two classes of cars, both of which run simultaneously in the same races:

TCs include Acura TSX, Audi A4, BMW 325, Ford Focus, Honda Civic, Mazda 6, Mercedes C230, VW Jetta,

GT cars include Cadillac CTS-V, Chevrolet Corvette C5 and C6, Dodge Viper, Porsche 911 GT3.
The **Grand American Rolex Sports Car Series ** also has two classes:

The other class in GAC is Daytona Prototypes, which are not based on street cars. As in the Speed World Challenge, both classes run in the same races at the same time.

So there’s no shortage of racing with production based cars. BTW, all of these series race on road courses, not ovals.

As AskNott said, the term “stock car” refers to the early days of racing. Today, Jeff Gordon and Jimmie Johnson aren’t driving anything you’d find in an ordinary car dealership, but back in the Forties, it was a different story.

Back then, bootleggers would buy ordinary cars (the same old Dodges, Chevys, Fords and Plymouths that their law-abiding neighbors owned), strip them down and soup them up to be as fast as possible- fast enough to carry a haul of moonshine across the county line before the sheriff could catch them.

Many/most of the earliest stock car racing champs were bootleggers. And while NASCAR has come a long way since then (and so have the cars), the term “stock car” remains, as a reminder of the sport’s roots.

The stickers are a minor plot point in the new movie Cars, as it happens. Although not Pixar’s best, the movie is well worth seeing.