Go take apart an oil pump. Inside looks like this:
View attachment 1715477537
High volume pumps are bigger. More mass to turn, more volume of oil to pump. Takes more force to do that, all coming from the shaft. Think of it as a torsion bar, with added load. More twisting. You can only twist the intermediate shaft so much before it snaps and hardened shafts helps that. The gear hardness is more for the wear with the cam.
This link has a little info about gear material in the last section:
Distributor Gear Selection - Getting It Right Is All About The Details
Basically you want the shaft gear softer than the cam gear so you aren't wearing into the cam and compromising the cam's strength.
