There are two ways to build a seat on a carved show-wood chair, and the choice decides how the chair ages.
The quick way is a fixed seat: the padding is built onto the frame and the fabric is stretched over it and tacked underneath. It looks crisp, it is cheaper, and it welds the upholstery to the chair permanently. When the seat wears - and it always wears first - the whole chair goes to an upholsterer, the carved frame gets handled, and the finish takes its chances.
The slower way is a loose cushion sitting into an upholstered frame. It costs more, because you are effectively upholstering twice: once for the frame that receives the cushion, and once for the cushion itself.
What it buys is independence. The cushion can be turned, so it wears evenly instead of hollowing in one place. It can be lifted out and cleaned properly rather than sponged in situ. And when it is eventually tired, it is recovered on a bench somewhere while the chair stays in your room, unhandled and unmarked.
The carved frame is meant to last twenty years and more. It makes very little sense to fix a five-year textile permanently to it - so we do not.