Why are most senior positions filled by lateral transfers, and it's difficult to promote from within?

Let me give you an example so you understand.
I am an experienced director who has been working in this department for many years, with my own team internally, a group of capable veteran colleagues;
externally (whether at the bureau level or district level), I also have my own connections and channels, and when something happens in the bureau, I have the ability to communicate, coordinate, and solve problems externally.