You can guess why fast food meals are cheap! The quality is lower. When meat is minced up small you are unlikely to notice. Buy enough of one meat product for mass consumption and the price will go down, and that's what these chains do of necessity. Try buying lean beef mince and make your own burgers for your family and see if you can do that for a dollar or so! The strict hygiene a chef will insist on won't be possible in meat processing factories and chain outlets.
Fried foods and sandwiches are affordable because of the cheap ingredients they use. They also don’t take a lot of time to make. This would save on labor costs. To make food more affordable restaurants would have to serve the same meals over and over again. Other restaurants may serve a today’s special which would be a new kind of meal. They would not know how much to buy because they do not know how many people will order it.
At fast food restaurants, they know how much food to order based on their weekly inventories. That brings the cost down. Next time you go through the drive-thru of your favorite fast food restaurant, remember the food may be affordable, but it could risk your health.