The real question you should be asking.. why are there no mass shootings in schools, malls, or movie theaters with actual machine guns? The answer is not only surprising, but also a stab against the firearms community. But we can easily cover it up with finger pointing, assumptions, and disbelief.
Come again? For starters, you can't hit a durn thing with one. Have you ever had the pleasure of firing one? Tons of fun, that's about it. A friend of mine has a (completely legal) little .22 machine gun. I'm a very big, very strong man, and it took a lot of strength to maintain control of it, and accuracy was less than stellar, to say the least. I can imagine a full auto AK would be a real handful, and seriously hard to hit anything at all with.