They don't, its just the level of skill, or lack of it these days, that many seem to have. Just because they cant shoot, doesn't mean others cant.
The 5.56 loses that "ballistic edge" at around 300 yards or so, but it will still punch a hole in you at longer distances. If you don't think I cant hit you at 500 yards, that would be a mistake on your part. And though its not as explosive at those distances, Id be willing to bet, you'd notice it, and not be happy about it.
The three round burst was a bean counters idea of saving money. They didn't, and from what Ive seen, still don't, want to waste time and money on training and ammo for general troops.
FA fire is all about trigger control and knowing when and where to use it. Its not wasteful in the least if you learn to use it, and its a very simple technique to learn.