The answer is because when ANH was made, Lucas didn't have the idea that vader was really Anakin skywalker yet, so there was no need fopr Owen to give Luke a new name because there was no threat. Then, between ANH and ESB, Lucas had this idea of Vader being Luke's dad, which created the question of why Luke's uncle never thought to change Luke's surname. As for ObiWan's piss poor alias, I have no explanation. Personally, I believe that Ben was his real name and ObiWan was a name given to him by the Jedi. Of course, that idea has been destroyed by the prequels, but I don't watch the prequels so it's ok.