Remember that these cultures don’t define Islam. They are simply taboos in some societies, not related to religion, but often the result of misusing religion as a tool for mind manipulation.
As for problems, some couples have to deal with disagreements among relatives, where they fight over who should marry their daughter or son. In the end, those who get through all the drama often end up living happily ever after. It seems that marriage is made difficult here.
I get the impression from what you say, that many times people marry for economic interest, the land or the amount of money that that family has, and of course to maintain that inheritance within the family and prevent external lineages from enjoying those benefits.
Or marry from the available limited options! :)