Thank you very much. You already used the rule in your feedback. It’s “why it EXISTS”. Another more instance: he CRIES, she SEEMS and so on.

Well, it can sometimes help to give information about singular vs. plural (she cries, they cry), and in general verb conjugation (which is absent in Chinese) is just simply a linguistic feature of many European languages, including Latin from which English is in large part derived. I could argue similarly that I shouldn't need to say 一个月 because the measure word is superfluous, I should just say 一月 like I say 一天. If that's too far I could argue that there is not a need for any 量词 other than 个, because English very rarely uses these kinds of words (you'll see them often when describing groups of animals, or in "a pair of pants").

If you're interested, there has been an experimental language which attempted to be as simple and logical as possible in the hopes of becoming a universally spoken language. It's called Esperanto. However, as long as languages exist they will inherit from those before them and have quirks that are sometimes useful and sometimes just complicated.