Why are automatic weapons illegal in the US?

Automatic weapons as we usually think of them are really semi-automatic.

That is, you fire one bullet for each trigger pull, and the weapon charges itself with a new bullet automatically. These are not assault weapons, though most politicians think they are.

Real automatic weapons will 'spray' out a stream of bullets as long as the trigger is kept pulled. These ARE considered assault weapons.

They were outlawed largely because of their use by gangsters who had more firepower with them than the police did. Automatic weapons are pretty dangerous because of all the firepower they emit. The killings that you read about where "Assault" weapons have been used, have NOT used assault weapons, only semi-automatic weapons.