Or are they just in homes where everyone else values reading and access to text (and adults to explain it) is plentiful?
Reading is an obsession with my toddler right now, but if I had not helped him learn, I am sure he would have given up trying. Talking to my friends (almost all older children) many of their children were similar. Some encouraged it, some didn't want their children to learn the 'wrong' way so didn't help. All are good readers now.
I was like my DS is now. My mother taught me with Peter and Jane and I was fine lol.