We were going to have titanium ones made with a group buy. A couple people brought up that maybe it was made to shear just in case something got caught in it to keep from possibly damaging anything else. Then most people started having second thoughts. So I just dropped the group buy.
It was deff designed that way for a reason. Not just to cut costs, because raw material costs are most likely very similar (basically negligible in the grand scheme of manufacturing parts for this motor).
I'm guessing a plastic gear will transfer less harmonic vibration to help avoid the fragile oil pump gears from shattering at high rpm.
That's just my guess thou, I'm an M.E. But have no design experience with this particular design of oil pumps....