I think it's just a "grass is always greener" mentality that people fall into because they're terminally online and only see all the problems in the US that don't exist in other countries. The problem is that everywhere has its own societal issues that most Americans are unaware of because they spend all their time fixating on what they don't have rather than what they do have, which is quite a lot when you think about it. People who say the US is a third world country have likely never been to a country where you can't drink the tap water without becoming violently ill, or where there's no running water at all. 

There are countries run by dictators who at any given moment are fending off at least five different rebel groups, who are all at war with the other rebel groups and regularly go around massacring each other's villages and forcing the kids to become child soldiers; but people in the US have to pay high medical bills if they break a bone, so the US is clearly a third world country. Sure our healthcare system needs improvement and it would be nice to not lose your life savings over a heart attack, but if that's the worst problem we have as a society we're doing relatively well overall. It's all about perspective.