The first person shooter formula is highly successful as well, but there's been significant innovation in that genre over the last twenty years. Nobody thinks that OG Doom was the pinnacle of all that could be achieved with that style, despite it being hugely successful to this day.
I think the argument that GT sells well and so therefore it doesn't need to improve is weak. I can see the appeal of that if you're a developer, you can look at how GameFreak has handled the Pokemon games if you want to see that taken to a logical extreme. It's gotta be real easy to hack together the same old assets and squeeze out another installment for a fat paycheck. But as a consumer, I don't understand it. If a good game is good, then surely a better game would be great.
To be clear, I don't think that you're specifically making that argument, but it's the undercurrent behind what many people say when they echo sentiments like your first sentence. And it's how we end up with people hoping that we get a game that's 2001 game design with shinier graphics.
Why would Polyphony, with a game series that sells ten million copies per installment, want to take a bite out of iRacing, with an active userbase of ~150,000? That's like McDonald's saying that it's main competition is Joe's Fish 'n Chips. Gran Turismo exists to sell hardware for Sony, and that means appealing to more people than the turbonerds like me who play iRacing.