I think it's mainly because in modded Minecraft, you can build all sorts of contraptions and the like and there's a great deal more to do (make high-end weapons and armor, explore other dimensions). In Vanilla, you can collect a few things but there really isn't a whole lot you can do in terms of contraptions that do tons of stuff for you, so once someone has enough diamonds for their equipment they're probably going to focus on building something more elaborate than a square house. In the modded game, rather than focusing their efforts on making the house look great aesthetically, they focus on the functionality and what is inside. Particularly for tech mods, houses are more like industrial factories than houses.
I generally build boxy buildings myself, although I generally try to replace blocks like cobble with better-looking blocks and making things look a bit cleaner once I have the resources and tools for it. I usually end up starting my base underground, though, starting inside a hill and expanding downwards as well as laterally, only building up and above the ground later on.