It's certainly much easier to not have to make it yourself. So I'd certainly go with laziness or certainly time poor. Stupidity, maybe. I do think people know that it does them no good, but I think many aren't truly aware exactly what the ingredients are in them. So it's easy to put it to the back of your mind. As long as the effects aren't immediately apparent, then it can't be all that bad, surely?! Sickness and obesity can creep up slowly.