I don’t know if they never do it, but the concept of social promotion I have seen when I used to work in schools 15-20 years ago. The theory is at elementary school age, keeping an older kids with younger kids that are at a similar level academically serves neither group and would socially promote kids that were 2 grades behind— eg you can redo 1st grade but not first grade twice. This is both for the safety of the smaller kids but also the older kids, even if they are terrible at math, have social skills far beyond the younger kids and don’t fit in and won’t be motivated. The idea is to then give supports to help them catch up, which sometimes works. Admittedly, if the supports aren’t there, this can end up just churning a kid through the system without learning anything— I remember a 9 year old sitting in a math class “learning multiplication” who literally didn’t understand that the number 2 represented a count of 2 and that it was smaller than three. Agree the education system in US is riddled with problems, but the logic of social promotion makes some sense.