Why are german cars so unreliable?

I own a BMW z4 and for years now, I can't seem to figure out for the life of me why German engineers (who are supposedly some the smartest people next to the Japanese auto maker) would go and do something like build a thermostat housing and water pump impeller out of plastics. It seems like common sense to build these parts out of metal, after all, consumers pay a lot of money for these vehicles, why all the steering and electrical problems too? I mean come on! How hard is it to build something that works properly? I've read an abundance of reports from power windows failing, to over heating, to air bags, to convertible top failure. But a Honda accord at a fraction of the price will never have such gripes? Can anyone answer this from an educated standpoint? (Please do not tell me how reliable your Audi is followed by the mileage on it, no smart-alec answers either ).