Pressure times flow is power.

Aircraft systems are high power systems that do a huge amount of work moving controls, extending and retracting flaps and so on.

To keep system weight down they go for high pressure systems, at the limit that seals and pipes can handle without blowing.

At this pressure limit they still need a high flow to provide enough power for the job.

And please note that the fluid is going round and round, out and back all the time.

The limit here is that if you try and save weight by using less volume of fluid then fluid velocities will increase, producing more heat and energy loss, even possibly fluid cavitation and mechanical damage.