Since when are monks all martial arts warriors? Sure, Shoalin monks teach martial arts, but Buddhist monks in Japan certainly don't.
Wait, you are arguing that the term "monk" is wrong because a term that doesn't fit all real life monks. And yet the wrongness depends solely on your arbitrary assertion that in D&D "all" monks are "martial arts warriors". Doesn't the answer to your question reside in the question itself?
If you went up to a Shaolin monk and said they can't be monks because they are not the same as Buddhist monks, then the foolishness of your comment would be self-evident. Applying that reasoning to D&D monks is just as foolish. Nowhere in the rules does it state that "all" people who carry the title "monk" are martial arts warriors. You seem to have injected this into the game. And, that's a shame. Just as in real life, the term "monk" applies to different things. A particular form of adventuring person is one of those things.
For the record, in my games there are a lot of clergy at various churches, and many of them will answer to the label "cleric". Not all of them carry maces or know how to cast Cure Light Wounds.
Only D&D derivitives cast paladins at all. Everyone else calls them knights.
Ok, since your whole argument here is based on being completely anal retentive over the meaning of words, I'll start with the fact that in a strict sense you just said that no one ever calls anyone a paladin. So you are wrong.
If we instead agree that the term paladin does, in fact, pre-dates D&D then we now know that there are people who were called "paladin" and there were reasons for this label being added in addition to "knight". Its the old All Paladins are Knights does not mean All Knights are Paladins. Thus, the term Paladin does have a more precise meaning that Knight and is useful. Paladin refers to this subset. So you are wrong.
Also, the D&D "paladin" has certainly evolved into a more inclusive term, not of knights in general, but further contrary to your claim, it includes a range of divinely called warrior types who would not be considered knights. For example, Joan of Arc is an often-cited paladin archetype and the is Batman a paladin debate is a recurring theme. Though maybe you should write to DC and explain to the them that a guy in tights who knows martial arts clearly can't be a "Dark Knight".