Presumably because people are eating things that, underneath the sugar, are pretty much tasteless.
Consider the American Chinese staple of "sweet and sour pork." What does it taste like, without the sweet and sour sauce? Well... it's deep-fried boiled pork. Would you eat plain deep-fried boiled pork? Even as a side-dish?
If we take away the artificial flavors, a large number of (usually cheaper!) dishes simply cease to exist. Palatable food—especially palatable fast food—would get a lot more expensive. What of the people who work all day, end up too tired to cook, and yet don't end up with enough money for healthy food?
(If the answer is "we don't want those people to be in that situation in the first place", then try solving that problem—if you manage it, I have a feeling it would largely solve America's obesity epidemic as a byproduct.)
5 years ago I went on a diet where the only thing I did was to look at all the ingredients and nutritional values in everything and if they wern't there I didn't buy it (most burger places will supply it if you ask).
It is APPALLING what gets added to everything in order to be just-a-bit-better-tasting than the competition. Your brain doesn't know why brandA is better than brandB. And it's not just sugar, salt and fat are also adjusted.
But you don't NEED those flavors. You have to wean your body off the flavor. Foods have subtle and delicate flavors that get masked by the assault.
You can't hear the sounds of a bird chirping or a trickling stream if you are next to a rock concert.
It is my own observation that supermarket/farm vegetables are often bland compared to "backyard" varieties. Most people have no idea how flavorful a simple tomato can be until they've had an "ugly" heirloom tomato left on the vine until ripe.
The problem with making food taste more sugary, with or without artificial sweeteners, is that as a society we are growing too accustomed to hyper-stimulating foods. Why can’t people just live with the fact that every food doesn’t need to be loaded with sugar and instead open their palates to the notes that come from natural flavors?
It’s like people want their hair to be blown back every time the fork goes in their mouth.