Illustrations of history

Without modern metallurgy and and the associated industrial manufacturing base it's a lot harder to build a single mill to effectively use all the potential energy. A multiple mill setup like this allows you to extract most of the energy without reaching the limits of how much force you can put through machinery made of wood (the shafts and bearings are likely also wood, not just the wheel buckets), stone, and low strength metals.

Overshot wheels, as the ones displayed in the reconstruction, are more efficient. Theoretically, I suppose, they could have made it one massive structure like a modern dam, but that would have been a lot of architectural effort for no real increase in efficiency.