In America, it's generally still legal to assemble your own firearms from readily available components. Folks mostly do it for legal or financial reasons, although some people do just want to be able to say they built it themselves.
In California, assembling your own weapon is frequently the easiest way to legally acquire an AK-style or AR-style rifle. In order to comply with state law, it is necessary to assemble these rifles in manners that out-of-state gun owners find ludicrous. Due to the limited commercial options for completed rifles, Californians tend to build their own rifles from parts.
Of course, some folks like to build rifles in a group setting, also known as a "build party". Here are some recommended build party guidelines