I'm sure they would absolutely LOVE to do this, but they won't for the simple fact that the moment people can't buy a new Nintendo console KNOWING they'll get a new big console quality Zelda, or a big new console quality 3D Mario, Smash, etc etc etc is the day Nintendo's console sector truly dies.
You may think "well, all those people would buy anyways once the Zelda is unveiled/released!", but it's actually important for fanboy moral early on to be able to have that idea they can project all their wildest fantasies onto.
Once the actual Zelda game is shown off, if people didn't already hype themselves up, it's suddenly easy to shrug it off and decide they're not going to bother buying it afterall.