That's what it has going for it. Fast Food can hit three key elements, which is why it plays so well with those with less money. It's cheap, it's convenient, and it's loaded with stuff that's as bad for you as it stimulates your tastes.
An interesting comparison point is that of ground beef at the market. A dozen ready-made burgers from Burger King at fifty-cents will cost six bucks (and tax). That's about the price of two pounds of moderate quality beef at the grocers or 1.5 lbs of the leaner healthier stuff. The thing is that the premade burgers have a convenience factor that the latter two--which have only a price/quality trade-off--don't have.
As much as I dislike them, when the food doesn't taste bad, costs me fifty-cents a burger, I can't argue with the convenience. On the other hand, after going on my own version of the Super Size Me diet for a week, I've again sworn off fast food burger joints--with the exception of White Castle of course.