Because the moment you have it in legislative form, if you need to add to it, you can't. There is no more authority. You lose a certain flexibility if you do it that way. This is about an implementation. The core reporting obligation is clear; it is about the threshold you see. The types of payments, the types of payees, and the breakdown is what you see addressed administratively.
If we were to legislate this, again, we'd lose that flexibility. We would need a legislative amendment to add additional information fields or break down requirements.