Well, I wouldn’t say that “developed” implies countries aren’t trying to improve themselves further, any more than “rich” people aren’t trying to get richer. You can call it whatever you want but at the end of the day you need to have some terms to describe poor countries where people die of treatable diseases because they lack infrastructure and money, and countries that don’t (as an example). Global South is just a euphemism for the same but a lot less intuitive and accurate.

There isnt a developing country on the planet that isn’t trying to be considered developed.

And I dont think this is a particulary American term. Third-world, sure.