The best way I've found to think about this is that, physiologically, there's nothing stopping your brain or mine from being the brain that holds a GM mind.

However, in order for that to have happened, we needed to have our brains manipulated a very specific way as children, when plasticity was at its highest, into learning how to think the way that best fits with being a chess GM. Since that didn't happen, our minds have settled into patterns of thinking that are very hard to now undo or rebuild, and likely not as well suited for chess.

Could we tear down how we think about problems at a fundamental level and rebuild ourselves to think like GMs? Yeah, probably. Could we do this while maintaining any amount of day job, social life, or literally any other aspect of a natural human life? No, probably not. Changing not just your opinion on a topic, but literally how you think, seems pretty insanely hard to do after childhood/adolescence.

I don't love the phrase "mental capacity", maybe "mental configuration" is more accurate.