Why is fuel used as a hydraulic fluid?
The primary reason is availability -- the high pressure fuel pump is running whenever the engine is, while the airframe hydraulic systems may not be available in case of an accessory gearbox failure, hydraulic leak, or maintenance shutoff.

Also, this keeps hydraulic lines from needing to be plumbed deep within the bowels of the engine -- this has benefits in that the hydraulic system is confined to the accessory gearbox fire zone (where the engine driven pump lives), instead of having to run in several engine fire zones.