A single unbroken seat on a long sofa is a comfort problem before it is a style one.
Divide a seat into three cushions and each one is supported by the platform directly beneath it, which is straightforward. Make it one long cushion and the middle has the furthest to travel before it finds support - so a poorly built version sags in the centre within a year and everyone slides toward the middle.
The answer is in the platform below. Our upholsterers graduate the springing along the length: firmer through the centre section where the unsupported span is greatest, and easing toward each end where the frame is close by. The cushion above is then filled to a matching profile, slightly fuller through the middle, so that when it is compressed by someone sitting the top surface comes level rather than dipping.
Sit at either end and you feel the same seat you feel in the middle. That evenness is the entire reason to build a single-cushion sofa rather than a divided one, and it is the reason most single-cushion sofas are disappointing.