Because it is a lie. Children need to learn math in a logical progression, not be introduced to concepts early on. They need to learn how to count to 10, then 100, then higher. Then they need to learn how to add. Then they need to memorize multiplication tables, etc. Everything builds on top of another.
I don't have children, but I have nephews. I can see the results of this New Garbage when I help them with their homework. If a kid answers half the questions right and get half wrong, consistantly, then you know they are just guessing at answers and do not understand the concepts or the questions. Especially, with simple concepts like: Round these numbers to the nearest 10's: 35, 54, 12, 143. And the kid comes up with 30, 50, 20, 140.
Well, I agree with you. They do need to learn the basics first. More importantly, I think they need to be held back until they have learned them. No social advancement and an emphasis on the basics. This works best for most I guess. But, I still think kids are far more able to learn advanced concepts than we give them credit for.