because oddly enough the game ends after the ending. CRAZY I KNOW!!!!!!!!! but it actually has a good ending, letting you play afterwards would ruins that. it gives you a huge warning before the point of no return. its impossible to miss. anyone who complains about not playing after you beat it is a complete moron.
My gripe with FO3 WASN'T the fact that you couldn't keep playing after the ending, but rather that the ending was lame.
The game has a fantastic ending that brings things to a close, it was an ENDING. The means the game ENDS. I don't get why people want to play after the credits. Why?! What's the point?! Explore, do your stuff BEFORE ending it.