They wouldn't need a community manager if they made a better product.
That’s not really true, community managers are supposed to be there as communicators between community and dev. Even when things are going great it’s good to keep people up-to-date with what’s happening internally.
Good community management helps to relay the fans’ wants and needs to the dev teams so that they can improve the game in the right ways. They also make the community feel listened to and offer assurance that the devs are pushing the game in the right direction.