Because, there have not been any fundamental changes to the function of a semi-auto firearm and a full-auto firearm since the NFA. Guns are old technology. Other than materials, there is nothing high-tech about pistols, AR-15's, AK's, Mac 10's, or belt-fed semi-autos and full-autos.
About the only high tech "gun" that
might not be covered by the NFA would be Metal Storm:
This is not something that can be bought, found on the black market (
or any market), or made in the back yards of hobby machinists. So, we don't need to change the NFA to address it.
https://youtu.be/wKlnMwuCZso