1. I'm hardly going to use my computer from a couch.
2. Even when gaming on the TV, I still use a chair because I'm near-sighted and my couch is too far away from the TV.
I can only speak for myself, but it's because there are more types of chairs than couches. Computer chairs, massage chairs, recliners, etc. What do you have with couches, futons and sofa beds? Not as versatile, and I'd assume I still can have an actual bed to sleep on in the presented scenario.