为什么 I 跟 am 而非 is 一起使用？

Conjugation
As in most Indo-European languages, the English copula is the most irregular verb because of constant use. Most English verbs (traditionally known as "weak verbs") have just four separate forms, e.g., "start", "starts", "starting", "started". A sizable minority (traditionally known as "strong verbs") have five separate forms, e.g., "begin", "begins", "beginning", "began", "begun". "To be" is a very special case in having eight forms: "be", "am", "is", "are", "being", "was", "were", "been". At one time, it had even more, including "art", "wast", "wert", and, on occasion, "beest" as a subjunctive.

Etymology
From Middle English am, em (“am”), from Old English eom, eam (“am”), from Proto-Germanic *immi ("I am") from Proto-Indo-European*h₁ésmi (“I am, I exist”), a form of the verb *wesanan. Cognate with Old Norse em, emi (“am”)……