The reason is that they are deadly afraid of mass layoffs being leaked beforehand and causing the stock price to slide or a bunch of negative press about it, so the people who know aren't telling anyone but the VIPs in the company about it until the last possible minute.
They are getting a bunch of negative press for rescinding job offers. This doesn't seem any better. Better would be just not hiring people you don't need and not chasing growth like an unmonitored heroin addict.
