Because that would allow people not like them (i.e., common, law-abiding citizens) to be able to own things that they feel they alone are entitled to. And we can't have that, you understand.
There don't appear to be many fully automatic rifles in the inner cities where they're most likely to be used if available, and I have a hard time thinking that's a bad thing.
I'm a member of the NRA and own many guns myself. I'd prefer that fully automatic rifles be highly restricted in this country.