Market and loss limiting. The warranty on BMWs is on the higher end of most. I don't think it is a function of not trusting the design, it's more about what (nearly) everyone else is doing.
FWIW, back when Hyundai was first selling cars in the USA, they were terrible and the only way they could get people to buy them was to put a long warranty on them. There's a perception that by deciding to shorten a warranty, that admits there's a problem, so it would be harder for them to shorten it now, and build it into the price of the vehicle.
2011 535i x-drive GT, 2014 i3 BEV