Language is a product of history and does not necessarily have a logical explanation. The answer is "because" and the theory is usually that numbers bigger than

Language is a product of history and does not necessarily have a logical explanation. The answer is "because" and the theory is usually that numbers bigger than the ones we use our fingers to count were not usually dealt with in pre-the literate societies from which our languages derive. That seems reasonable, since without writing it is difficult to keep track of large numbers. But it's just a theory!