They actually really know how it works, but it's too dangerous to give the public details or they'd build their own time machines and go back in time and insist the writers compose a better story, or not make a particular mistake, and all too often that's just subjective desires and not objective requirements, so it's still impossible to please everybody all of the time, and the next thing you know it, you have a flash time mob where you didn't before. And then you'll trouble. Then the group who don't care about Who drop in just to see the time mob or cause further mischief. It's a nasty business, time travel. Best to keep it under wraps.