“Why would you allow someone else to tell you what it is you believe in? You don’t get to decide what being a conservative/liberal means. Someone else decides that. You aren’t part of it. So why would you let whoever that is tell you what you should think?”

the short answer is because the agree with it.

The better question is asking them whether they want to agree with something someone else said once, or whether or not they want to have their own belief foundation, their own principle system, and their own way to derive an answer to a problem.

The problem with modern day politics is that nobody, almost nobody is willing to engage critically with the problems at hand, to determine a real, functional solution to the problem, or at least, the best possible solution they can come up with. Everybody is perfectly fine and content with saying whatever the funny man on the screen tells them, and that's the end of the story.