I think part of the reason is because [...] fast food restaurants, already have the factories and stuff that are designed to make the unhealthy food. I assume they ask customers from time to time, "Would you like to see more vegetables and fruit on the menu?" but there doesn't seem to have been a great demand for healthy food, or [fast food restaurants] would have changed by now. All I can say is, the owners of the fast food chains must be raking in their money, and probably have no interest in changing their policies.
On another note, beef is actually one of the most inefficient meats to produce. On average, for every 1kg of edible meat in a cow, it ate 8kg of roughage. Pigs have a ratio of 3kg of feed to 1kg of meat, fish about 2:1, and chickens 1.7:1. The most efficient things to produce are plants.
[Please note we have removed references to specific fast food restaurants from this answer, just to be on the safe side. - AB Editor]