Corroded Golf Mk2 GTi clutch bearing cover

Found the gearbox oil on my Mk2 GTi really low. Hadn't checked it for a while so guiltily filled it up. A few days later it was empty again. Found clutch bearing cover next to filler plug had corroded allowing oil to leak out. Oil loss had been hidden by dirty gunge. No mention of it in gearbox section in Haynes manual (Haynes a constant disappointment these days). Happened to find it mentioned on internet. Levered cover off (it was so corroded it broke up) and fitted new one. Why did VW fit a mild steel cover instead of normal screwed alloy cover? Gearbox could have been knackered. Might have shortened its life as it is. Big black mark to VW there. Interestingly, sump and steel coolant pipe both holed by corrosion some time ago. Did VW use some cheap steel?