A 10x developer is not one which can solve a problem in a few hours instead of a few days - It's hazardous to think about it like this because, often, developers who solve problems quickly tend to rush it without thinking about long term implications.

A 10x developer might actually complete individual tasks slower than the average developer would. You can only know if someone is a 10x developer by looking at the long term (multiple years) - A 10x developer understands the business domain very well and so they can anticipate a range of possible future requirements; this means that once they've written code, it doesn't require any major restructuring/refactoring later as new requirements are introduced in the future.

The reason why there aren't many 10x developers is because predicting the future is difficult. It's for the same reason that there are few good CEOs; it requires vision; the ability to make realistic projections about the future.