The second reason is that machine guns are strictly regulated in the United States. They are not illegal to own outright, though many people think they are. The National Firearms Act of 1934, or NFA, established a series of restrictions and taxes on the purchase and ownership of certain firearms that Congress associated with crime. It also created a registry for lawfully owned machine guns, which were subject to various licensing requirements and other conditions.
The NFA also provided the first and current legal definition of what counts as a machine gun. Congress defined one as “any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger.” The full definition also covered any components that can be used to easily build a machine gun, as well as “any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machine gun.”
Congress closed that registry in 1986, meaning that only the individual machine guns that were registered before that date can be lawfully purchased and owned by civilians today. This placed machine guns out of reach for most people. Not only are they impractical and excessive in their own right, but the artificially constrained supply means they are pretty expensive as well. Most of them survive today as antiques, collectors’ items, or family heirlooms.