'Twas a joke, my friend. 'Tis all.
I wouldn't put it all on him. I favor the notion that most language devolves based on ease. It's not only easier to remember just adding an S to an existing word, instead of fully altering it, it's also phonetically a much easier practice on the mouth musculature... oofs or uhfs compared to ooooovvvvvszszszs lol