Digging used to be much more expensive, and calculating pressure much dicier. The Romans used the pressure method over short distances, with reliable piping (noting that pipes under high pressure would need to be sealed with earth or concrete), but over large distances, it was easier just to make a giant-ass water bridge and ensure it was sturdy.
Why did the guy in charge of this use a giant bridge instead of the bottom method? Is he stupid?
Digging used to be much more expensive, and calculating pressure much dicier. The Romans used the pressure method over short distances, with reliable piping (noting that pipes under high pressure would need to be sealed with earth or concrete), but over large distances, it was easier just to make a giant-ass water bridge and ensure it was sturdy.