It is cooperative. But since the DM is the one providing the campaign they can choose who to share it with.
Again, that seems to be your experience, but it is not a universal truth.
Case in point, D&D Club at my school is open to all, and I cannot deny kids the right to play unless they are breaking school rules.
In my home game, I am the DM, but the story is built in cooperation with the players. Their backstories are the basis of the major story arcs, and I bend the world to allow for their character ideas. Nothing is set in stone. If a player wants to play a concept that requires significant changes, we find a way to make it work. They also add story details and description if they want.
Aside from being a contrived way to build a character, alignment also tends to ask the DM to act as a sort of morality police, especially as it was traditionally conceived. That style of storytelling is not for me. D&D functions absolutely fine without alignment - better, IMO - but I'm glad it's there for those who like it.
As I originally posted, one of my current players at home is kind of old school about it and uses it for his paladin. That's his choice, so I just roll with it. In my beginner campaigns, I just tell the students not to worry about that box on DDB, and the game proceeds without any significant difference. You have to tweak a few spells. Alignment really is vestigial in the current rules.
Edit: oh, and many folks have mentioned that they love it as an aspect of setting - games set in a grand conflict between alignment factions, Planescape, etc. Again, not my jam, but so what? Others love it and that makes it good.