That's a good question. I believe it's just an outgrowth, or continuation, of the position we held at the end of the Second World War, when much of the developed world apart from the western hemisphere was in ruins, and we 'assumed' the role of world policeman, aid worker, and democracy (re)builder. I'm not so sure it's necessary anymore.
One of our greatest issues is the amount of money in our "defense" budget, much of which goes to defend other countries, most of which could assume that burden themselves now.
I fail to see how a 'mutual defense' treaty with Luxembourg or Portugal, or first-world, developed country, is an even swap these days, as it may have been in 1950.