The sensor works by sending out a pulse of ultrasound and listening for the echo. The time between sending the pulse and getting the echo is proportional to the distance of the reflecting object. If the sensor were to have an analogue voltage to read, yes it would be easier to use but it would be a much more expensive sensor as it would have to do the sending, timing and converting to a voltage. The arduino's processing power can do this and so you have a lower cost sensor.