But most kids are not in this exetreme range. They themselves
- can't know what needs to be done and at what time
- can't find appropriate material to study
- can't setup high enough goals for themselves
- can't make their own plans
- can't measure their own progress
- can't tell whether they are on a wrong path
- can't correct themselves quickly if they are on a wrong path
- can't figure out what is blocking their progress
- can't persist if their progress becomes stagnant
Therefore, they need some external help from a coach.
Even Bobby Fischer, who was famous for self-teaching himself chess and Russian (because he wanted to study Russian chess books), has a mentor, Jack Collins. All world champions and other top level tournament players hire seconds to help them train and practice.
Everybody has blind spots s/he can't see by him/herself. For beginners, there are many, maybe too many.