Yes, you can. But you would have to sit there for an awfully long time. Inevitably, when you are at your desk or lounging on the sofa, you will move your legs even if you stay in roughly the same position. The difference on an aircraft of course is that unless you get up to go to the loo or strecth your legs, potentially you might have your legs in exactly the same position for a few hours and this is what causes the clot. You are also in an extremely cramped postion which has a double whammy effect!