I am not sure it is a standard, much more to do with convenient information being propagated over the past 15+ years.then why is it that a low phosphate reading is recommended and that standard across the board?
Also under certain conditions 0.04-0.08 range will work, but you need to ensure there is constant supply of PO4, so for example in a natural thriving reef there are lot of fish and they constantly produce coral food (fish poop).
Calcium reactors also constantly provide PO4, using live rock seems to work with low PO4.
As mentioned don’t just focus on PO4 and pour a magic bottle of PO4 in your tank and hope it works. It likely will not work.
P/PO4 is critical to corals and from my experience fish provide the best type but other things like trace elements, alk etc… also need to be available.