Probably because sustained heavier-than-air flight requires an engine. So same reason the car was not invented earlier - a coal-powered steam engine is too heavy.
It's quite good at steady-power output. Throttle response is poor.
Both automobiles and aircraft operate with quite variable power requirements. Though at least for long-range air travel, once the take-off / climb portion of flight has been completed, cruise is typically at a fairly constant setting.
Sure, but beside the point. There are many historical examples of heavier-than-air flight (kites, gliders etc.), but without means of self-propulsion it is not very practically useful.
