Because heaven forfend we use brute memorization to learn our addition and multiplication tables. Why, that’s just white male patristic hegemony trying to make our kids smrt. Because it’s a much better idea to start them off with algebra, that’s the inclusive way.
I get that you want kids to know why math works, but first or third graders have, like, miniscule reasoning skills. Explain it to them later, in pre-algebra, when they can care about whys. In the meantime, it’s easier and quicker and more useful to just accept it on faith that 13-7=6. Or not faith because if you have 13 oranges and someone takes 7 you can count and see that you have 6 left