Ikea's aesthetic is not just some designer's idea of what nice-looking furniture is, it's actually designed to be flat packed. The raw materials that go into the product ARE cheaper for Ikea (particle board vs. hard woods), but not by leaps and bounds (unless you get in to the more exotic hardwoods). Flat pack is also cheaper to warehouse and distribute, and your custmers can come and collect it and build it themselves. Mid to high end furniture often has more intricate joints and curves, and is probably cut by machine, but assembled by hand in a factory.

TL;DR: You'd have trouble producing furniture in other, more traditional styles that could also be flat packed and assembled at home by the average consumer at a reasonable price point.