Often called "Panglossianism" as per Voltaire's character in Candide:

> Dr. Pangloss was the pedantic old tutor in Voltaire's satirical novel Candide. Pangloss was an incurable, albeit misguided, optimist who claimed that "all is for the best in this best of all possible worlds." So persistent was he in his optimism that he kept it even after witnessing and experiencing great cruelty and suffering. The name "Pangloss" comes from Greek pan, meaning "all," and glossa, meaning "tongue," suggesting glibness and talkativeness.

The idea that there's always a good reason for every societal practice that we currently observe is arrogant too, just in a different way.