因为古英语是这样的。

古英语不光有单复数，还有主格宾格属格与格工具格，阴性阳性中性。中古英语没研究，早期现代英语还有第二人称单数的-st，单数第三人称是-th。比如：how darest thou spake such words How dare you spoke such words He hath been banished.He has been banished.