Kitchen out of date? Gut the kitchen module and replace the parts. Home theaters are out, and home gyms are in? Gut the theater module and reconfigure it into a gym. Sticks-and-drywall construction can be demolished and rebuilt in practically a day. Room "pods" can be torn down, merged, or separated at will, with little impact on the livability of the rest of the house. There's a lot more (ostensible) flexibility in an amorphous blob of room modules than there is in attempting to maintain a coherent, symmetrical, balanced architecture. The traditional house is a fixture; the McMansion is a living, changing, multicellular organism.
McMansion developers aren't selling houses; they're selling services. All of this runs parallel to, and feeds off of, the macro-social trend favoring interiors over exteriors. People care more about kitchen appliances and countertop surfaces than they do about architecture. It's crass, tasteless, and economically silly. But it's the prevailing culture. And it's designed to keep selling things.