Why? National mutual self-interest is probably the single most long-term stable and historically workable approach to foreign policy, economics, and so forth that I can think of.

It's not an approach to the world. The US government isn't a beneficient association that is organized to make things better for people in other countries at the expense of US citizens -- it's not an aid organization or a charity or something like that. The interests of non-citizens don't have any representation in it (it's like every other government in that regard) -- it's just the wrong organization to do something like that.

In 100 years we’ll say the same thing when Planet Earth says “it’s not our responsibility to promote the interests of the Mars people”. People just inherently suck.

I'm not saying it's a bad thing for Americans to do things for other people in a way that costs them something, even something considerable; but it is certainly strange for their government to do so by moving jobs abroad and otherwise promoting the interests of the working class of other countries as the comment I was replying to mentions.