Frankly, because the team that sets policy is different from the team that makes the buttons. The policy has lots of exceptions - e.g. some categories can't be returned, like groceries. To disable that button, they'd have to check for all the exceptions and constantly update it whenever a policy changed.

They did change to automatically approving returns a few years ago, but with an opt-out ability. My guess is either OP's product fell into an obscure exception to that rule, or OP opted out, or something similar.