Because people hold to irreconcilable conceptualizations of reality (broadly speaking), and, as part of this, oftentimes irreconcilable conceptualizations of what is right (both generally speaking as well as in regards to specifics, e.g. socially, politically, economically, etc).
People frequently avoid the uncomfortable prospect of even civil discussion with others who hold to beliefs different from their own, instead preferring the comfort of likeminded individuals who affirm their beliefs (something made exceedingly easy with the advent of the internet) thus perpetuating the divide and stifling the prospect of compromise where possible.
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