...because no one can answer that question until it happens. After WWI they asked the same thing, came to the same conclusion as you, and it's one of the reasons they almost ended up a part of Germany.
The only difference if they are wrong today vs. yesteryear, is a conventional war will most likely be over a lot quicker. As we've seen the world is constantly getting smaller and even if you cut the typical procurement/production length by a factor of five, no one is going to have time to ramp up production of MPA, carriers, jets, bombers, etc.
It amazes me how ignorant the rest of the world is to how much of the stability enjoyed over the last twenty years is thanks to the United States. They criticize our military spending, accuse us of being warmongers, and tell us we should spend more on social programs, yet don't realize their ability to spend so little on defense and at the same time enjoy such security, is thanks to us. As we've seen in Libya with cruise missile and bomb shortages, they'd be hopeless in any sort of conflict, let alone a major one, without out our aid.
Our very presence is the only reason the question "Why do they need to still be so big?" when referring to a island nation that already has no carriers, no MPA, no conventional bombers, and less than 200 attack/fighter aircraft, isn't totally absurd.
People have just accepted this current world as a peaceable one without ever identifying the reason. If/when the United States relinquishes the role of world police man they are in for a rude and abrupt wakeup call.