Because if you took less at a time your body will have already processed some of the previous dose before you take the next does. You shouldn't eat 5 pounds of black liquorice in one sitting because it's toxic in that amount, but you can safely have that amount spread out over time.

Imagine taking 6 ibuprofen all at once instead of taking 1 or 2 every few hours. That would absolutely not be a good idea.

Everything on earth that can be consumed has a dosage that is too high to safely take all at once, even water. You're not just pouring this stuff into an isolated beaker.