[cmrdporcupine]

This. There's a reason why they almost always put the triple-car garage out in front as the most prominent architectural aspect: "Hey, look how many cars I can have."

Better than "McMansion" even is "Faux Chateaux"

(Having a big spacious home is not something I'd safely criticize as I have 6 acres and a house a bit too big for the family. The simultaneously ostentatious and ... ugly... yeah)

[bombcar]

It's often other reasons, usually because the house is designed with a single garage and there's an "upgrade" button available.

It also makes the house fit on a smaller lot if the driveway is straight to the road.

Most if not all McMansions are builder's specials designed to be sold and upsold; often if you go to the builder's site you discover that the lowest end version of it actually looks pretty sane, all things considered.