A few reasons. Some good. Some not. Some by choice and others by having a weak identity that can be molded by others. You might as well ask why all cultures cant seamlessly mesh together. Its due to irreconcilable differences and a rejection of that which one opposes. There is tribalism and an inability to accept that which one detests. Then there is how humans grasp a patterns and seek simplicity. Their definition of themself and others is by default something that is swayed by external influence. Its easiest to point and say theres your problem or this is just how things are or how a group is instead of breaking down everything into nuance. While practical to talk like that some people are shallow and dont think beyond that or with any greater complexity. They lump things including themselves and others into groups and decide this is how things are. Should a person begin to perceive themself as a group member instead of an individual that is part of a group they will often adopt the beliefs, traits, ect... of what they perceive the group to have. They mold themselves into a living stereotype of one degree or another. Matching their own beliefs of how they perceive the group standard and possibly matching the beliefs of others. To get metaphysical its kind of like defining reality and seeing it change because people are limited and shaped by expectation. Lefties share so many traits with cults. I cant imagine it to be anything other than an absolutely artificial creation. Due to the purity tests and censorship, attacks on those outside the cult dogma, the peer pressure, one sided extreme bias, constant stream of belief affirmation, and highly selective memory the majority of them can only be perceived by others as a group member when they conform to a rather specific group standard. Through that then identify as a member. Despite them often saying otherwise there is no great room for individuality. Everything beyond that tiny box is called right wing. After constantly being called right wing people associate that with themself. They see a highly antagonistic and manipulative group that is clearly not them and is opposed to others also called right wing. This helps cultivate an us and them mentality and that solidifies group identity. When people are constantly told you are this thing that is not like us they adopt that identity. Right is deemed the opposite of left and red is deemed the opposite of blue. The roles for those who play the game are predefined and individuals are constantly being told this is the box you are in. If people see no reason to oppose it they go with it. When someone feels like the role matches them and they already disdain the other team of course they will often willingly adopt that role. Be it because they picked it for themself or because they are impressionable. Tis the nature of man.