A rate that the average family can afford is not the same as (the effectively impossible) setting a rate that every family can afford.
The number you threw out as the average affordability was 75%. What about the other 25% of children/families? If both parents are working, or if it is a one parent family, home schooling isn’t an option. And if one parent stops working to home school some of the children (which isn’t even a possibility for the one parent family), their income goes down and their ability to afford tuition and everything else goes down further.