@deesenaughts
Because western/developed countries provide opportunities for people to grow and perform at their best, while 3rd world countries lack those opportunities.

To make an analogy, imagine if Mozart was born in a poor farmer family, that could never afford to give him any music teaching/training. Maybe he would have grown up to be a decent farmer since he probably was pretty smart overall, but he would have never gained the historical fame he has.

Now as for why those countries lack those opportunities, it's because they didn't manage to develop to the same level as, let's say USA. And to understand why this is, I recommend the book "Why Nations Fail", because the answer is not that simple.