Because the “third world” doesn’t mean what people think it means. It’s cold war terminology. First World is western aligned nations, Second World is Eastern bloc, Third World is nonaligned with the eastern or western nations. FBFW it meant nations that generally weren’t powerful or wealthy enough to be of interest to either East or West, and that poverty often meant they really had a lot of infrastructure and other problems. That’s why we’re not “Third World”, it’s a geopolitical alignment, not a quality of how we treat our citizens and infrastructure. So we’re First World even if our country is turning into a shithole in some ways.