I'm sure it is a process of indoctrination that starts from a very young age, just assuming the opinions you hear are facts when you're a kid and subconsciously carrying a lot of these prejudices with you until you actually rigorously study politics and form your own opinion based on rationality rather than dogma. Although there are certainly many exceptions, especially in academic circles, I think a good general rule is if someone can define their political leanings in one or two words then they don't really know what they're talking about.