Because air coming off the back doesn't flow over the rest of the car.
Why not stick them on the back and aim to make more disturbance for cars following...
Because more disturbance would mean more drag.
If you create more downforce you'd generally create more disturbance anyway. The aim is to create more downforce and less drag, but the two go hand in hand and so there lies the art of aerodynamics.
@duncs started
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Because air coming off the back doesn't flow over the rest of the car.