The main reason is their parents raised them that way. The minor reason is their parents were too strict with them and their children rebelled. The third and least common reason of all is they researched the issue for themselves.
As far as I can tell, no one is raised as a libertarian.
Here's something for you Team Red and Team Blue authoritarians to ponder: Don't you think it's interesting that when adults research politics, economics, and history, they almost always become libertarians?
Adults who research religion thoroughly almost always become atheists or agnostics. Adults who research government thoroughly almost always become libertarians or anarchists.