Same reason a 4x4 is a poor choice for highway use only.
Now, with a car, it’s not that much of a problem. You’re got an engine to compensate for your poor judgement. It’ll cost a bit more in fuel, that’s all.
Now, on a bike, YOU are the engine.
It’s YOU who’ll be paying. Either in sweat, or in lower average speed, or both.
The tires and the upright riding position will really hurt your attempts at going fast.