Yes, there should be parenting classes, because some people need some help to know how to parent. Parents who have abuse or neglect cases brought against them by the government need somewhere where they can go in order to demonstrate they are trying to improve their parenting abilities. Parenting classes can help people like this learn basic parenting skills.
Should there be parenting classes? Yes there should be... and there are. Parenting classes exist now and some folks attend them voluntarily because they are interested in being good parents. But the vast majority of parenting classes are those to which courts assign parents when something has gone wrong in their handling of their children or those that are held in prison to which inmates are assigned or may choose. It is unfortunate that more people do not take parenting classes before they become parents and when they are new parents instead of the current situation in which many times parenting class ends up being a remedial measure when there has been a problem.