if all "you or I" wash at home then that's the actual "orders of magnitude more runoff waste" compared to if everybody did it at the car wash. Car washes have to purify the water since it's actually possible to do it there since all the washes happen at the same place
Car washes probably induce demand for washes. After all the fact you are paying $7 to wash in 10 mins is because the good old driveway method is too inconvenient. Once again the dose makes the poison. Whats my runoff relative to the rest of the area? Small. Whats the runoff of a concentrated source like a car wash? Massive relative to what is running off in the rest of the area. Dramatically shifting the contents of that runoff in terms of what they are.
