Physics stops you. Ever ride a bicycle? 25 mph is easy, 30 is hard, 40 requres special gear or a very pumped up body, and 50 is nigh impossible. Why does the effort get so steep for such small speed increases? Air.
Rolling resistance is the dominant factor when your car is going slow - that's why they're so hard to push. But at higher speeds, the ruling factor is aerodynamic drag. That's because rolling resistance is fairly linear. Aero drag is not, and is at least a second-order factor -- it comes on like a freight train at higher speeds. In fact I've heard coal trains that can do 40 mph loaded, can only do about 35 empty. That's because each empty coal car is a big scoop, catching the wind. And rolling resistance is close to nil for a train.
Obviously, that aero drag is balanced against the power the engines can apply against it. They could get that empty coal train up to 70 mph if they put four times the locomotives on it.
You're talking about cars, so it's a matter of how much the batteries can supply and the motor-controller can push without taking damage. Or since you're assuming fuel-powered cars, it's about how much air the engine can move through. Adding the correct amount of fuel is easy. Airflow is decided by tuning - intake/exhaust ducting/porting/resonance, valve size and numbers, camming, stuff like that. (Also the engine redline (max RPM), but if your ducting/resonance is tuned so raising redline will make a difference, that engine is going to run quite poorly on the street.)
Gearing can also be a factor. I had a car which struggled at its top speed. Fourth gear was too tall for it to be able to accelerate. Third gear put it past its horsepower peak, so the faster you went, the less horsepower you had. The gearing was all wrong for speed, but it was excellent for street-driving fun, refinement and MPG, which is what I paid for.
A typical non-sporty street car like a Ford Flex is likely to hit physics limits of aerodynamic drag before it hits a point where the computer would rev-limit or MPH-limit.