There's a huge amount of positive feedback loops involved in education so if you can give your kid an start that gives them a tiny leg up on their peers it's likely that they will carry that lead all the way through to adulthood.
It's the same arms race we see in athletics. Kids that start school a year later (and are bigger) have better athletics outcomes.
Few people really are naturally exceptional at anything. However, if you've managed to shift your kid's performance curve so they look "gifted" compared to the other kids then your kid gets the best the education system has to offer and this investment of resources compounds in a way that gives them better opportunities which at scale correlates a lot with better outcomes.
Basically parents that have the time/resources/will to game the system in favor of their kids have kids that out-compete the kids of parents who don't. Water is wet. More news at 11.
My 4y/o is learning to read at the moment, and the difference between the early reader phonics books he gets from school and the stuff I read to him at home at the moment is night and day in terms of story and interest. There's a real market for some really good authors to produce better phonics books.