They still do because they still are? I'm hard pressed to think of a better way to move water than creating a gently sloped channel(this includes any pipelines that don't use pumps, by the way) for it to follow from where it is to where you need it. Many places that don't have sufficient local groundwater are still supplied by aqueducts to this day. If you have to pump it, then you're paying for the channel and the energy, which is worse than just using gravity.