It's a lovely summer's day, you go to the river for a picnic, you take the family and a portable BBQ which you then set up. You cook some steaks or chops or sausages, and lay out an array of salads and sweets, and prepare for lunch.
But even before you finish unpacking the word goes out on the fly telegraph that there's another human eating in the area and every fly for twenty miles turns up for the feast.
You spent all the money, prepared the food, packed it in your car, drove to the flies' picnic spot, and prepared them a feast toward which they contributed absolutely nothing but gate-crashed, and ate their fill.
Sort of like what dodgy said, that's how flies are. They have way different life styles than us, and we can't even speak to them so we can't be sure they aren't intelligent from their standards. And we might be stupid from their standards too. And I bet all the monkeys think we are total idiots, too.
Flies are just doing their natural thing. I don't think they can tell the difference between our food and theirs. So, they just buzz off to whatever's edible. Plus they look ugly, but think. Wouldn't flies think we are ugly too?
The intelligence of a species is limited by its size. As flies are very small, their brains are very small.
But this question really comes down to what we mean by 'intelligence'. Like all animals, flies are intelligent in ways that humans aren't, as they have developed skills and behaviors that are beneficial to their survival. For instance, we have found that fruit flies make complex decisions about the amount of alcohol they consume, and actively seek out foods containing alcohol if they host endoparasitic wasps (a certain amount of alcohol kills these parasites). In other words, they self-medicate.