Why did Romans use elevated aqueducts instead of pipes to covey water?

Water will flow without the aid of any power, if the aqueduct has a 2 degree slope.

At this angle water will flow steadily without surging. So if water had to be carried across a valley, then an aqueduct was built. or through a cutting with a water channel.

If water had to be lifted then an Archimedean Screw was used, driven by a mule or donkey.

So over varying terrain , hills, valleys etc., aqueducts were constructed with a slope of 2 degrees. to make the water flow, without power and without surging,