Walking into another country and claiming it as territory really isn't as easy as you'd think.
I mean look, we literally have shit poor countries in Africa, that are RIPE for the taking.
With the major Ebola outbreak, attcking Africa would be inhumain and quite frankly, really dumb.
Even for the African countries that aren't plagued with disease, conquering smaller countries for their territory would probably turn out really badly. Not everyone likes war, ya know.
Lastly, keep in mind that European countries pretty much owned all of Africa, but that quickly changed after World War Two. Independence is important to people, and keeping territory that doesn't want to be your territory provides a pretty big problem, especially if someone decides to start an uprising or something of the sort.
We got middle Eastern countries too incompetent to do anything to protect themselves.
Middle Eastern countries are a major oil source for like, everyone. If you're gonna attack the Middle East, they won't be the only enemies you make with that decision.
We got the afghans looking like idiots growing their weed.
America is already waging war for Afghanistan. Jumping in to take Afghan territory would mean going to war with America, which is, in itself just a really horrible idea.