Check out this recently constructed McMansion in Minecraft. Here is a description of the structure:

Finally, it’s here! I have built an amazing McMansion!

This grandiose house features:

• A large entrance and foyer,
• a large living room with a high ceiling (and a balcony of the second floor hallway),
• multiple smaller rooms that could be sitting rooms, a dining room, a kitchen, etc.,
• a back porch, and
• 10 bedrooms! Gee whiz!

I have built two other McMansions before (both on the Iciclecraft server), but this is by far the best one.
Feel free to paste it into your own Minecraft world. However, if you use it in multiplayer, please credit me as the builder.

Sounds like McMansion features. The only thing missing here is a full neighborhood of mass-produced McMansions. And the tags for the post reinforce the McMansion idea:

Tags:Mcmansion, Mansion, Manor, House, Grand, Large, Big, Grandiose, Land Structure

I suppose the quick answer for why someone would build a McMansion in Minecraft is because they can. Perhaps they like building houses. But, to intentionally design a kind of home that is generally viewed negatively begs for a better reason. If you could build anything, why a McMansion?