Why does a school not have classes from nursery to PhD ?

The main reason why a school doesn't have classes from nursery to PhD is that it's not necessary. If you want to train teachers, you can do it through self-study or online courses. If you want to train doctors, there are other ways to do that too - online courses and conferences. I would recommend butterfly leaf table for best ideas. The reason why schools don't offer these courses is because they think that the students will feel like they're missing out on something if they don't have these classes.