Because they teach you things that are absolutely counterproductive to think.
Remainders and "there are no negative numbers" are the two I can think of immediately.
Ah. I suppose it's all a matter of priorities. The whole idea of elementary school is to teach you basic methods of functioning and general definitions of things that exist rather than show you how to think, I believe, and to basically make sure that, whether you learn to think appropriately or not (not something that's particularly easy to teach, given the sheer number of teachers of varying quality one goes through), you can do arithmetic, write words and compose sentences, talk a bit and know some basic facts about life, plus have a decent grasp of socialization. A modest goal, of course, but, in my opinion, one should not expose children to too many unnecessary complexities, even if, looking back as adults, we think of the simplifications as obstacles to further learning.
Because it's completely and utterly against everything they're going to teach you in no less than 5 years, they teach you things that you're going to have to forget as part of the curriculum and believe it or not MATH IS PRETTY IMPORTANT FOR A LOT OF THINGS
On the bright side, one could learn about adaptability and questioning one's beliefs that way. And math is pretty important for a lot of things
for a certain, some would say small segment of the population, while basic arithmetic, which elementary school simplifications help learn, is a lot more crucial. And if you don't learn algebra or geometry in middle/primary/high/whatever the hell kind of school, just like if you don't learn Spanish or singing, you can pretty safely deal with life without it, it's no big deal. You have to pace the information and layer white lies together with basic truths. It's not like knowing that negative, irrational and complex numbers exist, or strictly proper mathematical notation, or any other piece of such trivia really helps you in any way when learning arithmetic - it just makes things more confusing instead. In short, just because you'd find it convenient as a thinking human being right now doesn't mean it'd help a developing mind in any way. But I'm hardly an educator myself, really, so that's merely an opinion not backed by any real research.