I don't understand why people would still have a desktop these days. You have to sit at a desk to play or basically do anything. I wouldn't get anything done if I couldn't mod from my couch. If you don't play anything requiring high end graphics (shooters and the like), there's really no point.
It is about money and time. Laptop needs to be replaced every two years while desktop every 5 year. Also performance good laptop cost about as much as average desktop so it is more expensive as well.