Because we can build it bigger and better. People don't care if their PC draws 10W more, so long as it performs. If you go mainstream, you will however see things getting faster and shrinking. That E7200 of yours has a tiny 107mm^2 die and more performance than any Pentium D. As for chassis and the like, all you're seeing is enthusiasts around here wanting more room. Go to an average person's home and odds are any desktops they have are mATX, and quite compact by our standards.

What will Asus say about my EeePC RMA?