Why not help make some of these places better places to be? One poster said it himself; mexico is crap.
Aside from the political and economic reasons not to, the US does give a lot out. There are many, many non-profit organizations that do improvement projects overseas, and the US government gives out a ton of aid every year (enough so that some people at home would rather see the money put to use cleaning our own country first). Unfortunately, a lot of those funds end up going through corrupt government channels, and the aid never reaches it's intended destination or purpose. If you'd prefer, the US could go around and depose all the corrupt governments guilty of this, but somehow I think the rest of the world would frown on such action.