It's a simplified analysis that Japanese verbal suffixes encode politeness, though even Japanese grammarians analyse them as such but it doesn't match descriptively how they are used.
Other analyses more so analyse these forms as creating distance between oneself and the statement, lending them a more impersonal tone. A similar effect to how “would” can be used in English to say, say, “That would be the case.” instead of “That is the case.” which also sounds less direct and more polite, because it creates distance.
Japanese native speakers have been known to use the “plain” form even in communication with superiors when such distance is not appropriate, such as when expressing genuine concern.
https://www.tofugu.com/japanese/da-vs-desu-in-real-life/
Japanese grammarians have analysed many forms in across many lines which do not really reflect how Japanese native speakers descriptively use them. “gendered speech” is another one that is commonly criticized.
Another similar case is Dutch, where textbooks will tell there is one way to say “you” to superiors and one way to peers and inferiors. In reality the “superior” “you” simply creates distance and sounds as though one is speaking to someone one is not on personal terms with. It's really not much different from “Mr. Johnson” vs. “John” and while the former is more commonly used against a superior, it can also feel inappropriate for a superior to use the latter even when talking to an inferior.