"it's not a monkey it's an ape" is so funny because. apes are monkeys. and then you'll hear someone like "i know phylogenetically yes but thats not how it's colloquially used." but like. clearly it is. someone just used it.
People are offering a lot of theories as to why people care, but I think it's important not to leave out that the historical and christian perspective on biology is that life is organized in a hierarchy with humans at the top and everything else ranked by proximity to humans. Apes are closer to humans than other monkeys are; thus, they are "higher" on the "pyramid of life" than all other monkeys.
This philosophy is still one a lot of people have in the modern day; fish and insects, which are distant from humans, are treated as "lesser" animals, sometimes not even animals at all, while mammals and birds are valued over reptiles and amphibians. In this philosophy, the idea that apes are monkeys is perceived as a slight; to call an ape a monkey is to treat it as "lesser" (see: every talking cartoon chimp ever), and also, implicitly, an insult to humans (because we connect with the ape and consider it "greater" because it's close to us).
but also to the original point. when you "correct" someone that an ape "is not a monkey"... on what authority are you doing it? like there's no scientific information being conveyed, you only have old linguistic perscriptivism to fall back on. your pedantry is hollow.
