Obsidian has been pretty clear for the last several months that the game will have a definitive ending. Although you may not have finished the game in a way that resulted in major changes, let me assure that several endings result in the kind of world shattering changes that wouldn't be feasible to script into a post-game situation. http://www.strategyinformer.com/news/8998/obsidian-explain-why-fallout-new-vegas-has-a-definitive-ending a link that describes exactly what Obsidian was thinking. TL:DR Obsidian can't script every possible ending into the game, therefore the game ends.
And didn't the giant "END OF GAME THAT WAY ---->" warning suffice? Seriously, it's a giant warning that lets you know that you shouldn't continue unless you're really sure you're done with the game.
Yeah, I saw it! It's just still like ARGGG!!! LET ME PLAY though

I mean, they manage to when you wipe out a faction in the game, have them never appear on the "map" again, so I just don't get why they can't let you continue lol!