They don't return to Kobol because the scriptures said that any return to Kobol will be met with bloodshed (and that was proven in season 2).
Also this interview with RDM states that this is Earth. Not a pseudo earth or an alternate earth:
That planet is Earth? We’re not going to find out, “Oh, there’s this other Earth over here...” This is the only Earth we’ll see?
They have found Earth. This is the Earth that the 13th Colony discovered, they christened it Earth. They found Earth.