CCP Dopamine explains why.
What’s the alternative? Public “focus groups”? I hear that alternative a lot, but have huge problems with it. First and foremost, jst look at the forums and tell me that those would be useful. It’s a ton of shrieking from morons who don’t know what they’re talking about because they engage with only a minute fraction of the game overall.
On top of that, how would you discuss future changes without the NDA? CCP going to the CSM and saying, “Hey, we’re gonna do this change, what’s your perspective on how it would impact the game?” is pretty important. And because they can keep the CSM under NDA, they don’t have to worry about these discussions leaking to the public.