Because of trade secrets, of partnerships being formed and broken based on what is known, of financial implications, and many, many reasons.
Let's just make an example. ANP (fake AMD) are making a new GPU series with Dx13 or whatever support, something futuristic and whatever, something that has not been done yet. They work on this GPU architecture for a few years, put billions in R&D. Rent out venues and schedule a real world event to promote the new GPUs. And then Bob from My Dad's Game Studio goes on Twitter and reveals everything he knows about the new ANP GPU in details.
ANP had actually been planning to reveal two new AAA games during that press event, an exclusive right for which they've paid Steve and His Game Company 50 million $, and Tracy's Game Farm 30 million $, for marketing rights, support for getting new GPU-that-produces-smell features in the game, sending their own engineers to those companies to help with features, and other stuff.
Thanks to Bob leaking and revealing everything, there is now no point in holding the event, as the info is out there. The business relations between ANP and the two game companies are now strained, because the leaks came form ANP's side. The proper reveal of those games has fallen through, resulting in millions of $ of loss for all parties involved (not to mention the missed marketing and PR opportunities).
Thanks, Bob, a real 5 minutes of fame hero!