You aren't wrong, but I dont see that as a reason not to include open wheeler race cars. I dont see GT as a "road car" game, I see it as a "car" game, and the collecting, owning, modifying and racing of "cars". Since a large portion of it is the last one (racing), open wheelers are a perfect candidate that shouldn't be left out. Even from the collecting/owning/modifying perspective, I think it'd be cool to see something like a hillclimb championship, where you could build your own open wheeler race car, start with an engine, build a chassis and suspension around it, then upgrade it as you progress. Not to mention I'd love to own some of the old school open wheelers.
You could argue that GT is about "road cars" and not "cars" in general... but then I'd argue that many GT cars and LMP cars shouldn't be in it, because the links are so tiny from a realistic perspective. Well then I guess you could say GT isn't about road cars, its about "road cars and cars that are some what related to them in a even a very small way". But even then you have open wheelers which use street going components. So then I guess you could say "GT is about road cars and cars that are related to them in such a that they have a similar exterior appearance or such a way that the series they compete in once used road cars at one point in its history". In which case I'd ask why be so narrow minded and just open up GT to being about "cars, the passion of cars, the history of cars and the racing of cars", in which case open wheeled race cars fall into every category.
Plus, I enjoy racing open wheelers and think it'd be cool

Especially online, admittedly if I'm racing offline I'd rather race a touring car, but online open wheelers, particularly low powered open wheelers, are awesomely fun and produce great races where you'll be on the edge of your seat trying to push your car as hard as the guy infront without losing it.