Back in the 1940s and '50s there were no preschools. Some rich families sent their kids to "nursery schools," but those were not common.
And Kindergartens weren't required, so lots of children started First Grade without any previous school experience. Many churches included five year old children in their nurseries, so those kids hadn't even had experience in Sunday School.
Even after Kindergarten did become required, most of them were only for half days. Since most children had stay-at-home moms, five year old kids would be at home the rest of the time.
Today there are far more mothers with full time jobs than stay-at-home moms. Kindergartens start and stop at the same time as the older grades, and thousands of children go to after school programs in the afternoons.
Kindergartens used to teach the things kids now learn in preschool, so Kindergarteners learn the things kids used to learn in First Grade.
I used to be a substitute teacher and noticed that children who hadn't been to preschool were at a great disadvantage when starting Kindergarten. Many of them had missed learning basic skills like the alphabet, counting, and writing their names, and they hadn't leaned how to function as part of a group. It took them some time to learn to raise their hands and wait to be called on, walk in a line, sit quietly at their desks, take turns, etc.
Preschools are great for kids.