What position of saddle is best? Almost all cyclists who ride road bikes set the seat so high, and when they're in the drops their body can be almost parallel with the road, reducing air resistance. However,their legs can't reach the road from the saddle, and their head is pushed down and it could be dangerous in some situations. With a low saddle position, I feel comfortable and can easily reach the road from the saddle, and my head is upright. When I want to be more aerodynamic, I can drop my upper body and push my rear behind the seat. So why don't cyclists use a low position seat?
Road cycling position is generally a performance position (although not always) and can be viewed as a  compromise between three components:


Power - different positions will allow you to output differing levels of power depending on your biomechanics.
Aerodynamics - some body positions reduce your power, but can reduce drag even more therefore allowing for higher speeds.
Comfort - The position needs to be sustainable for long periods of time.


Depending on your goals you tend to weigh one category more heavily.  For example, endurance riding tends to be more upright which prioritized (3) at the cost of (2).

Saddle height tends to have the biggest impact on (1).  Too low and you do not use all of the power stroke of the quadricepts, too high and you start leaving the power stroke and can over extend your hip and knee. The most powerful and efficient saddle height leaves a small bend in the knee at the end of the pedal stroke. This lets you use your quadriceps to the end of their power stroke without over extending. You will not be able to reach the ground easily while remaining on the saddle.

Therefore to answer your question:


  When I want to be more aerodynamic, I can drop my upper body and push my rear behind the seat. So why don't cyclists use a low position seat?


You may be able to change body position to maximize aerodynamics (2) but you are still sacrificing power (1) and will be going slower than someone who first maximizes power (1) then gets as aerodynamic as possible. Finally, a low saddle position can affect hip flexion (making your hips and knees flex more) which can be further affected when getting the body into a lower position, therefore you could also be affecting comfort (3) which makes the position unsustainable in the long run.