Because with Hydraulics you can achive ratio's in the the 1000's to 1 in 1 go.

It's up to the builder to see how much power he wants to put in the system. But that's depending on what motor, and how far from the max efficiency the builder wants to go from the motor-power curve.

Basic formula:

pressure (bar)xflow (liter/minute)/500=Kw needed.

Example. we have a speed 900 drving the pump, with the max efficiency rating of 12V 13A=0.15 Kw

The most small , cheap gearpumps are in the 200 bar range.

(200*.45)/600=0.15

This give me a 0.075CC gearpump (each revolution is .075 cc*6000=0.45L/min) to be mated to the 6Krpm running Speed 900.
Unfortunatly, the smallest, affordable gearpump I ever had was a 0.17 CC=2.5 times to big. Easely solved with a reduction gearing. Pump will run 2400 rpm then.

Imagine a 50mm bore ram, stroke 100mm.
Power is 2.5*2.5*pi*200=3926 kg. Volume is 0.2 Liter

Flow is .45l/min. Filling that ram takes 27 seconds.

Half the pressure, and you can double up the flow.