Because the border situation will be much more of an adventure when the Chi-com and Russian military cross our southern, northern, eastern and western borders. That is, after they nuke us into oblivion. Isolationism did not work for America in 1941 - and its practice will do us no favors in the 21st century either.
Yet by then we won’t be recognizable as the USA anymore. Why even care about foreign affairs when you have a crisis in your own country?
Besides I don’t agree with your internationalist outlook anyway. There is little evidence WW1 or WW2 for examples had anything to do with us looking after ourselves first and trying to trade with all friendly countries, keeping a neutral stance. Both wars were well, well underway before we got involved.
There is as much evidence the US getting involved in every countries dispute actually does lead to more war. Take Syria for example and the Arab Spring Uprising partially started by Hillary and Obama that created ongoing civil war there as well as the mess Libya became as well as nearly a civil war in Egypt.
WW1 for example we had no more excuse to join in on the Allied Side than we did the central powers. The sinking of The Lusitania for example was a direct result of us choosing one side and supplying them with weapons and other supplies.
The Ron Paul’s have it right not the Bushes and Cheneys/ Clinton’s.
And this article from MSM source even:
https://www.usatoday.com/story/opinion/2017/11/27/clinton-ponders-2020-run-lets-not-forget-her-real-libya-scandal-glenn-reynolds-column/895853001/