Why are the planets in stargate so underpopulated compared to earth?

Normal people on earth don't know about the Stargate so don't have the option to get the hell out of here, yet the people on other planets almost all know about their Stargete, and how to use it ,even if they are less advanced than Earth, so maybe they went to other planets just because they could. I wonder what the population on earth would be if we knew we could just walk through a portal to another planet.