Why? The answer is easy, they have parents who don’t care if they can read.
My wife and I read to our kids before they could even sit up by themselves, prop’em up in your lap and read to them from picture books, novels, newspapers, etc.
The picture books were easy, they were kind of self explanatory; the novels and newspapers were more difficult because you had to slow down your reading speed so they could follow your finger under every word.
Then explain what it all meant.
Both kids were reading at the college level by the fifth grade, even had some teachers express concern about what they chose to read.
Had to tell the teachers to butt out, my kids were allowed to read anything they wanted to at any time they felt like it!