[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.