Why are reservoirs built on high ground?

Reservoirs are built on high ground to take advantage of gravity and the effect it has on the water stored up there. Gravity pulls down on the water, and no pumps are required to pressurize a water main hooked up to that reservoir. Yes, pumps are needed to lift the water, but the storage unit can be replenished as required. That translates to intermittant operation of the pump. The gravity fed water main is constantly pressurized by the pull of the earth on the water. A better idea from civil engineers. And a no brainer when one thinks about it.