Because it's part of the Fallout series. In the first two games you couldn't really do much after completing the main quest. I mean, you could run around and level up and possibly do some side quests you missed, but still... Well there you go, you could still run around, level up, and do side quests. Now sure, they added the endings they did for closure, but if they want to stay "true to the fallout series" they would've copied how the old ones were? And also, in Oblivion(Don't flame me for comparing it to oblivion), the open endedness was one of the most loved aspects of the game...why take that out in another very similar open world type game?