Because its tasty and filling! I love fast food and restruant food. For me it gets turned into muscle and then i have to eat again. Or else i will be too skinny. My body quikly burns off even the most greasiest foods! Fat is actually good for me aswell as protien. My body is a blast furnace for food since im always working and exersising.
If you mean as their main diet, not occasional treats, they may be unable to cook for whatever reason, they may be lazy, they might imagine home cooking takes a lot of time they think they don't have (or won't give themselves), or they might think it is cheaper than cooking for themselves.