Flat roofs can be functional. As the sq footage grows the area and volume needed for a pitched roof will grow much more. At the extreme of a 12:12 pitch you’d have 40% more materials for the roof and you’d create 50% more volume within the house. Additionally the space of the flat roof can be functional and used as an outdoor living space, unlike a typical pitched roof.
It is "modern" in architecture, but makes a lot of aesthetic choices over function (flat roof, no overhangs, a heated pool in a cold climate).