Part of it is age. Part of it is, I think, the fast food companies are lowballing what they can get away with serving in order to keep costs down. It's said that some Diné (Navajo) refer to Burger King with a word that means "just enough food to get strength from". I think that's the fast food joints' specialty: compromise the food till it's barely enough to tolerate and derive nourishment (calories) from to sell the stuff cheap and quick to a ravenous but indiscriminate clientele. With the passage of time comes more efficient ways to produce less delicious or satisfying food, so BK today is not as good as BK 30-40 years ago (which in turn is less good than BK shortly after its founding). Some burger joints e.g. Whataburger can differentiate themselves with higher quality, but they don't achieve the volume of McD's, Burger King, Wendy's, etc.

Back then, fast food compete with home made food, now it competes with another fast food.