Imagine if the banner said "This website is known to the state of California to cause cancer". Would you keep visiting the site?
Like if every time you went the bar, the bouncer asked "Hey, can I punch you in the face?". Would you keep going to that bar?
As annoying as the banners are, they actually aren't annoying enough to change mass-behavior.
You have to admit that if these same people can't be trusted to follow a simple "do not track" directive, humanity is in big trouble.
But I don't think it's that developers "can't" follow a DNT cookie. It's that they won't because it doesn't benefit their employer's financial interests.
Making a rocket that lands, on the other hand, does directly correspond to SpaceX's financial interests.