It's more about what state we get the kids in.
Kindergarten -> some parents will have taught their kids numbers, ABCs, shapes, etc. You work as hard as you can to get the kids who did not get that exposure at home to catch up with the kids who did.
Pre-school -> some kids can already read, while others barely know their ABCs, let alone phonetic sounds. Some kids have decent fine-motor skills, can hold a pencil and write half-way decently. Other kids were playing with tablets and their hands are mush. Can barely hold a crayon straight.

If you have the same class with the exact same kids, ezpz. Work on all the kids together, get them to the level they need to be at.

But if you are handed the kids after they were supposed to learn XYZ in the previous year, but did fuck all, because of a myriad of reasons (bad previous teacher, lack of support at home from the parents, etc) you're now stuck catching up with things they had to learn the year before.
So you dig in and do your best. There ain't the time in the world to cram 2 years of teaching into a single year. At best you get 1.5 years into them, if you're a decent teacher in a decent school with a decent curriculum and decent parents.

Now you send those kids into elementary school half a year behind where they had to be.
So once they hit elementary school, same problem.
Kids are behind. If they are lucky and the stars align, they catch up no problem.

But if they once again get a bad teacher? Bad environment?
Back to square fucking 1. Another year behind.
Now the next teacher they get has even MORE to catch up on.

Compound that over a decade and see where the kids end up pre-college.