Rounding every corner on a sofa is a decision about how a room feels rather than how it looks in a photograph. Hard edges register in peripheral vision - you notice a sharp corner without looking at it, which is why furniture with square arms can make a small room feel crowded even when it fits. Round everything and the same volume of furniture reads as taking up less space.
It costs more, because curves cannot be cut from straight stock. Every rail is shaped, every cushion is patterned individually, and the upholstery has to be eased round compound curves without a single pleat showing.
The place where that skill is visible is the transition on the back - buttoned across the flat centre, smooth where it wraps towards the arm. Our master Indian artisans stop the buttoning exactly where the curve begins to tighten, because a diamond grid taken one row too far into a curve puckers, and a pucker on an otherwise perfect back is the only thing anyone will see.