Because North Korea are always making empty threats and it's wise not to rise to them?
Yeah unfortunately the President is responsible for the safety of 300million Americans and if someone is threatening a nuclear attack they can't just "not rise to it", they do have to take that seriously when it's also being combined with missile tests and your intelligence is telling you they are fully capable of acting out their threats. Otherwise when thousands of people are killed, that's on you for not doing anything about it.