I suspect it's mostly synchronization issues. Trying to get two independent motors turning at exactly the same speed is likely a major challenge, and if the speeds are even slightly different then the car will pull toward the slower one. A differential meanwhile is a relatively simple and well-understood piece of technology that does the same job (uniform wheel power with slippage compensation) more simply.
Plus the cooling system is no doubt much simpler with only a single motor that's not surrounded by a big spinning wheel.
3 phase brushless DC motors are likely what is in use, they have very precise speed control as the rotor location is known at all times unlike a brushed DC motor, your problem would be apparent if different sized tires were used. I doubt there is any problem cooling hub motors as brakes have been air cooled for ever and sit inside your 'big spinning wheel'
The Wright Bothers weren't the first to fly. They were just the first not to crash.