There's a difference between packing light and packing for carry-on only.
For some it is all about how little they can take with them. Why use a 22in when they can use a 20in or even an 18in? I understand some people like that, and good for them. I am not one, though.
I do, however, pack just to have carry-on only. I have had too many instances where checked bags has cost valuable time, have gotten misrouted, or I have gotten stuck with a missed flight and without my luggage. I don't think those who check bags are evil, that is just how I personally prefer to pack.