Except they kind of didn't. When books were handwritten by scribes, every scribe used their own local variation of spelling to suit the wealthy buyer who had commissioned them. Later, it was in the best interest of typesetters for books to be readable by the most number of (wealthy literate) people, because they were creating more books on spec to be printed first and bought later, instead of creating each one bespoke for the buyer.

But ... then as now, there were all sorts of different dialects of English across Britain. People in the north pronounced things differently from people in Wales, Cornwall, London, etc. This was even a known problem at the time: what spelling to use when your book had to be saleable across so many different pronounciations? A lot of it was kind of an arbitrary choice, with most of the spellings matching London speech, and some matching northern speech.

I have to imagine that even at the time, there were people who read available books and wondered "Why did they spell it like that?" It's because printing made books "global" in a language and spelling landscape which was very "local".