Why is being an introvert considered to hinder personality development? Aren't introverts successful too?

What you are talking is a general perception about Introverts in this world. Its a perception and not a fact. In fact there are many introverts who are still successful in their occupation.

Right from Albert Einstein to Bill Gates to Warren Buffet or J K Rowling, all of them are Introverts, yet successful.

I need you to not believe in the generalized statements made somewhere and therefore it becomes a cause for your worry.

Being Introvert or Extrovert has not everything to do with your personality development.

By the way it is not a fact that people who are too talkative have good personality.