I'm not sure of the reasoning for the lack of parts.
It seems to me that it could be several reasons,
Building the body's and spools to tolerances that would allow spare parts would likely require higher quality tooling then is required for the current production methods,
Also the pressure in todays system is much higher then it was 10, 20 and 30 years ago, I can recall hydraulic systems with 800 PSI max pressure,
then it went up to 1200 PSI and progressively higher, now 2500 PSI and upwards of 3500 PSI as normal operating pressures, with the lower pressures o-rings could handle larger gaps then they can when the higher pressures, now we are seeing Teflon backup rings along with harder and smaller o-rings to handle the pressure.
I don't know if they are sitting with a box of bodies and a box of spools and selectivly assembling them possibly honing the body to accept the spool?? No idea.
To the best of my knowledge most of the valves now are made overseas, many it seems from Turkey.
