If people's beliefs cluster, then there must be a common cause of the beliefs. One possible cause is politics. But you left out another potential common cause, somewhere between your (b) and (c), which is that there is some single factual belief which causes the large number of specific factual beliefs, which, in turn, cause politics.

This is related to the existence of a left-right political spectrum. Why is politics one dimensional? Many people say that this is the result of the two party system, forcing people into coalitions. Do people wind up with factual beliefs supporting their entire party platform, including coalition partners whose interests are not the same as their own? If parties are coalitions, you might expect different coalitions in different countries. Correlations between factual beliefs might switch between countries. But I do not think this happens. And many countries, like Germany, have proportional representation systems that do not require parties to be large coalitions. Yet German politics seems pretty one-dimensional to me.