Because it's not simply a dream. To be exact, it's a memory. It's how his kids looked when he was taken away.
A thought about 'growing old', and yet dying as young people:
It's a dream. Age is, I suppose, relative.
The Evil Dead wrote:
I agree, I actually think I'd love the movie a whole lot more if we got that extra second of the totem toppling and then cut to black. I know I wanted it to topple over.
Yeah, that final shot was a bit of a turkey slap.
It's toying with us, but this is a Nolan movie. Puzzles are just as important as drama; considering the somewhat cool, distant (and convoluted) film The Prestige
That said, that Cobb continues to dream is far more satisfying to me. He lets go... he slips up. He's human and it's over. Cobb working out his problems has less heft to me, it would make the ending far altogether too tidy (if this is the real world; then the moment with the children is painfully on the nose.)
So yeah, while I think it continuing to be a dream makes more sense I'll also concede that's the far more emotionally satisfying answer for me.