Try reading some medieval literature sometime you guys; their spelling and grammar changes constantly throughout most texts.
Indeed. Rigid rules for spelling and grammar are pretty modern, though you'll find Roman and medieval grammarians writing books on correct usage (for Latin). But there's always a fluid relationship between spoken and written language, and a lot of scribes wrote words the way they sounded to them without worrying about it too much. And that continues to be true up into the 18th century or so.
I wrote my dissertation on 8th century Bavaria. There's a duke from the era whose name is spelled, depending on which text you're reading:
Odilo
Otilo
Ottilo
Uatilo
Uadalo
and probably a couple of others I'm forgetting.
In English, I blame all our troubles on the Great Vowel Shift of the 15th century. Before that, English was a phonetic language - you pronounced every letter and every letter made pretty much the same sound all the time. Monty Python was right, the guys riding horses in armor were K-nig-gts. You would speak the -e at the end of a word and a "long" vowel just meant you held it longer. A word like "extreme" would be pronounced "ex-treh-meh" and it all made sense. Then they had to mess it all up and change how they spoke, but keep writing the same, so that all of a sudden you have all these silent letters and all these letters that made different sounds, and though, through, cough, and bough didn't rhyme any more.
Honestly what I find hardest to read are modern authors who are trying to recreate premodern usages, like pretentious 19th century poets trying to SOUND like medieval English or Shakespeare or the King James Bible. Those all had a certain elegance to them, as long as you read them out loud and develop an ear for complex compound sentences. But imitators wind up generating the most pretentious convoluted muck.