Cost is a reason, but also there's social concerns. In the standardized system, all classmates share the same material, at the same speed. This shared experienc

Cost is a reason, but also there's social concerns. In the standardized system, all classmates share the same material, at the same speed. This shared experience disappears when every student is going at their own place, looking at a computer. This leads to the students being a bit more alienated from each other, and comparisons that go way past just a grade. When a classmate at the same age is covering material that someone else did three years ago, you will get the tension from both sides, in the same way that it's not all that great socially to be on a traditional school and take classes 3 years ahead. This issue disappears with all adult students, but around puberty, we are short tools whenever we don't have large enough cadres that we can just put all the kids fast at a given class all together.