There is leaving space for the DM to inject some creativity, and then there's deciding that you don't need to actually produce a complete product because you know your customers will do it for you anyway.
I mean, it's not like any of the published rules are mandatory. Just because they're in a book doesn't mean you need to use them. But them being in the book means you don't need to come up with your own half-baked, undocumented, and inconsistent "rulings" if you don't want to.
And, frankly, it's not a symmetric situation. Published materials are suggestions that, ideally, are crafted by experts and well play-tested that may be ignored if chosen. Unpublished materials cannot be opted into.
And combat encounter building is a core pillar of the game. It should not be a loosey goosey "rule of thumb". If anything, it should be the most reliable set of instructions in the book.