It's just the seated position. You sit in roughly the same position at home playing games, as you do (or I do) when you're at work.
It just feels too familiar.
On the couch, I'm a ways back from the tv, can stretch out on my massive couch.
I'm getting a new pc gaming rig in about 2 weeks and I'm entertaining the idea of hooking it up to the tv, but the idea of having to use a cordless keyboard and mouse and somehow balance them on my lap, perhaps on a tray to play pc games is really offputting.
Don't get me wrong, there are some games that I have to play on PC. But for everything else I would rather have a comfy couch.