My PC runs other games at a much higher quality and FPS
"games" is the important word here.
If you don't care for accurate physics, you can calculate the tyres on a separate cpu core than the chassis.
But if you want it to be accurate, you need to do it in the order of the real world physics and you can't split that into parallel calculations.
Imagine you would have to screw 3 pieces together with 2 screws.
You need to put the 3 pieces on top of each other, then insert the screws and then tighten them.
If you have 3 other people with you, they can't tighten the screw before you put the pieces together.
And if you think about it, sorting out how to put little things together with 3 friends isn't really going to accelerate things.
At some point the overhead to plan that would take longer than simply doing it yourself, alone.
Now if you would have another person planning everything beforehand, it would be possible.
But AC was developed by a rather small company.
Bigger companies have teams only optimizing code, trying to find ways to split things on multiple cores etc.
ACC uses about one core more. So there was some improvement.
And thanks to CSP, it was possible to optimize the rendering of the HUD apps, which boosts fps a lot when being CPU bottlenecked.