pipe fittings are used in high-pressure hydraulic systems

Threads on hose fittings are tapered and rely on the stress generated by forcing the tapered threads of the male half of the hydraulic fitting into the female half or component port. Pipe threads are prone to leakage because they are torque-sensitive - over-tightening distorts the threads too much and creates a path for leakage around the threads. Moreover, pipe threads are prone to loosening when exposed to vibration and wide temperature variations - certainly no strangers to hydraulic systems.
Seepage around threads should be expected when reusable hose fittings are used in high-pressure hydraulic systems. Because pipe threads are tapered, repeated assembly and disassembly only aggravates the leakage problem by distorting threads, especially if a forged fitting is used in a cast-iron port. Thread sealant compound, a potential contaminant, is recommended for pipe fittings, which is still another reason why most designers consider them to be obsolete for use in hydraulic systems.