Ok, it's simple, it's old, it's not unique, it replaces scripture, it ain't myth or allegory or metaphor… All these things you like about it.
Because of its simple truth
Why is it true?
When you have tools A, B, C, D... and so on, but find that only tool C has helped you and everyone else in overcoming every single moral conundrum ever come across, it would be safe to say that tool C is the only tool of relevance in solving problems pertaining to morality.
That tool, is the Golden Rule.
The rest, like the child slaughter-mandating 1 Samuel 15:3 that you choose to ignore as if it weren’t even part of the entire cafeteria you’ve called your own (even when you cannot explain what that verse, and others that were pointed to you, earlier, are in your scriptures for), are what tool C isn’t, when compared to the other tools. If tool C suffices, it is the truth. The rest, are garbs thrown around some truth. If you have tool C, you can throw out the other tools, as they aren’t of any use when it pertains to morality.