This is confusing correlation and causation.
One thing that people don't realize is the sheer amount of money required to become a "GM". You have to get a certain number of "norms" from FIDE and the requirements list is a PITA: https://chessgoals.com/how-to-get-a-grandmaster-norm/
If you don't have a lot of money and a lot of free time to go to a lot of chess tournaments around the world, you are not getting a "GM" title in this day and age. This favors children (few time constraints) with rich parents (lots of coaching along with the ability to travel).
The problem is that if you aren't a "chess prodigy" by 15 it generally isn't worth continuing to pursue--what does sinking the resources into being a "GM" get you at the end? If you're one of the "Chess Gods", you have entry into the tournaments anyway, and, if you're not, it doesn't really matter.