Sometimes it's because they don't study well, they haven't been to college or even high school. They want to make up for this regret through children.
But in fact, I think it is more because most parents in China love their children and want their children to live better, because they don't study hard. They want their children to change their destiny through learning and get rid of the fate of their parents' generation or even generations.
Because learning is the shortcut to success! Maybe they didn't regret not studying hard, but the hardships of life made most parents in China understand that the only way to change their life and destiny is to study hard!
Personally, some parents may not think so, but most parents should not be too different from this.