That's logically not true. It's not like there's a huge pool to choose from. There are less than 1800 GMs right now in the entire world.
And it also depends on what we mean by all this. Learning the pieces? Most children learn the rules of the game at some point, I'd imagine. That takes a way too many people.
Actually starting to try? There are a few examples I can find of people being around ~1700 rating when aged 18+ and making it to GM (Jonathan Hawkins, Jacob Aagaard, John Shaw)
Ye Jiangchuan didn't learn the moves until they were 17. Still young, but not crazy young.
> There are tens of thousands of adults learning the rules of chess each and every day.
How many find they have a massive passion for chess, without having a job, a significant other (or pursuit of one), any worries about money/bills/etc, or have children or family to take care of? That, and they can't have found their passion for chess during their childhood.
Probably almost zero people fit that bill. You basically need a lottery winner or trust fund person who doesn't care about having an abnormal life of chess with minimal upside in the small chance they actually reach the GM level.
But those are all not related to the fact that they have an adult brain, which is my point.
You're going to have real trouble finding an adult that is sufficiently motivated to become a chess GM, has an enormous amount of free time, willing to dedicate the next 10 years of their life purely to chess, etc.