Because the larger a BP gets, the less the math makes sense. To put it in perspective, you'd be feeding a 3000 gram female a 300-450 gram rat. Just no...
The "rule" (or, more appropriately, the guideline) was likely made after someone realized that it just happened to work for hatchlings. It's a nice way for new keepers to learn properly sized food until thy can eyeball it with more experience. It was not made because it is the proper nutrition for a BP based on anything scientific.