Why would Hydrostatic drive not go fast?

Hydrostatic drives can go as fast as you design them to go. The main problem is heat, the faster or harder you move the fluid the more heat you make. 5 mph on a lawnmower you can use a steel tube about 3 feet long, on a car going 50 mph you would need a radiator the size of the hood on the car your driving. Give or take.