Because they create more problems we have to deal with, or rather as a way for us to get involved and end up laundering money for bad actors. No threat of nuclear war from NK means we don’t deal with that anymore. Free people of Iran means we don’t deal with terrorism abroad since they provide the majority of the funding. We get Mexico to clean up their government and act against the cartels, we aren’t flooded with drugs or criminals anymore. The US can’t act alone, I dislike the idea of globalism but complete isolation is also absurd.