Languags don't get designed in a lab by a creator who comes up a consistent set of rules. Languages constantly shift and change as the people who speak them do. Languages borrow loanwords from each other, then proceed to mangle them. Slang arises, becomes part of the lexicon, becomes passe. Regional dialects drift apart but then mingle again.

And at no point does logic ever enter into the equation. Change just happens haphazardly.

There's a pair of concepts in Linguistics referred to as prescriptivism and descriptivism. Prescriptivism refers to trying to declare a set of rules for how language should be. If your teacher ever told you that 'ain't' isn't a real word, that's prescriptivism, and it's bunk. Descriptivism is just a best effort to describe how speakers of a language actually use it. If English speakers regularly say 'ain't', then it's an English word. The fun thing about descriptivism is that there will always be holes and inconsistencies, because not all English speakers are necessarily speaking the same way.

Compare the English we speak today from Ye Olde Englishe. Many words are now spelled or pronounced differently from how they used to be. Many old words have been replaced by completely different ones. Syntax has changed quite a bit. And if you go far back enough, English used to be written with a different set of characters from the Latin alphabet we use now. But this all happened so gradually you can't establish any clear dividing line to separate these languages, there's no date on which you could say everything prior was Old English and everything after is Modern English. And if you look towards the future, 100, 1000, 10000 years from now, English won't be the same as it is now either.