Because their parents looked down on them and told them what to do, and what they did with that was decide to do the same thing to their kids when they grew up.
Hopefully, you won’t make the same mistake of carrying on repeating the pattern.
Not all parents do that. And people do things for their own reasons, in their own ways.
Many kids do look down on their parents, especially when their parents mistreat them.
Some kids try telling their parents what to do, though that tends not to work out well, and trying to boss around other people tends to be a pretty miserable way to relate to people, unless they agree to it, for some reason.
In a healthy parent-child relationship, a child might not mind so much being told what to do, because there’s some mutual respect involved and understood.
Most (all?) parents tend to mean well for their children and think they are doing the best thing they know how to do. Unfortunately, they may also have some bad ideas, family patterns, and cultural programming.