Because that's how Apple's PR works, and always will.
Even the monumental iPhone was just rumors and guesses until the unveiling at their keynotes.
This is how you get more and more normal people to watch your famous keynote, not just geeks.
If for example Apple would give a list with all the products/specs they would present at the keynote, would anyone watch it? Probably just fans, but it would be pretty boring.
But since you have no clue, you want to watch it, and this way they can and will present it to you as being something revolutionary, and this way people focus on what Apple wants people to focus on, not on what you would usually focus on from articles (things like specs, size, bla bla).
Apple PR at it's best, always worked, always will...