Already answered earlier: Because it benefits everyone in ways nothing else can achieve.
Ok, then it does have a reason, a "because." We're still in logic with a because: to benefit everyone. Wouldn't you need to define "benefit" here? Is the benefit survival or is it happiness or material goods or evolutionary strength or cultural worth or my tribe survives to save the world...?
There can be differing ideas of what benefit means.
Whatever these benefits are defined to be, they become the greater value that the golden rule valuable as a means to achieve.