Re: Someone please explain why....
I used to think the same, but I don't think it's so cut and dry now. Without tumbling further down into an Inception analysis rabbit hole...I'll just say again that I prefer the "Cobb is in reality" interpretation, but I think Nolan was definitely going for an ambiguous kind of ending there. In fact Nolan said that he does have his own specific interpretation that he refuses to share because he wants people to have their own.
You could easily write a thesis paper on that ending (and books have indeed been written about the film), so I'll just leave my off-topicness at that, lol.