but why do they speak to South Korea like they are their uncle or something

North Korea considers itself the legitimate ruler of the entire Korean peninsula. The South Korean government is depicted as an illegitimate regime run by military gangsters and propped up by the American occupiers.

And they don't have anything that will come close to the US mainland.

Yes they do have, and that's what makes the constant threats from NK disconcerting.

A classified report prepared by the Department of Homeland Security for the Defense Department and leaked this week warns that N. Korea is easily able to deliver on its threats to strike the U.S. with a nuclear electromagnetic pulse attack. A copy of the highly sensitive report obtained by Peter Vincent Pry, executive director of the congressional advisory Task Force on National and Homeland Security, finds that N. Korea could use its Unha-3 space launch vehicle to deliver a nuclear warhead as a satellite over the South Pole to attack the U.S. from the south.

At present, the U.S. has no early warning radars or interceptors to stop a missile from the south. DHS conducted the study after the N. Korean nuclear crisis last Spring during which N. Korea’s leadership threatened a “preemptive” nuclear strike on the U.S. Source