Why do many parents work against their child instead of with them (when their child still lives at home)?

For example, when parents find out that their child is having sex at a normal age, they want them to move out. I never understood this kind of parenting because it doesn't accomplish anything. I get the "my house my rules" and "if you are so grown then move out"... but why do many parents need so much control? If a child is having sex (at a normal age), the parent should make sure they are practicing safe sex instead of kicking them out for having sex.

A different question: A child is separate from you, with different wishes, different dreams, wants, needs... Isn't it logical to work around the child rather than working around the parent? Instead of having the child conform to the parent's needs? Isn't the child more pure and unconditioned than the parent? Children do not have the logic to manipulate until around 7, so all of their emotions is true, meaning they have a reason for it. If a child throws a tantrum, the parent will punish him or her and give them the silent treatment, or take their toys away, or even call them names to make them stop. If the parent conforms to the child's needs, like they are supposed to, they would understand that the child is angry and throwing a tantrum because a classmate stole their favorite toy at school or maybe the parent forgot a promise they made. This is an example of working with a child instead of against them. It will also help the parent realize that their child isn't a stupid brat, that their emotions are valid, their child is a human, and that they do not own their child because each child has individual unique needs and wants that are different from the parent. It will also help the parent do this process with themselves.

So, How come many parents work against their child rather than with their child?