Why are so many capable people unable to lead a team?

Because strong individual ability does not necessarily mean strong leadership skills.
The core of leading a team has never been about copying your own abilities onto subordinates, but about tolerating and even encouraging a group of people who are less capable than you, allowing them to accomplish tasks in their own way, tasks that you could handle with minimal effort, and covering for their mistakes.
For most business experts, this process is counterintuitive to human nature.