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.
The design of the front forks also assists with stability - having some rake and trail means the front wheel has a tendency to self centre (particularly at speed).