Even in phlegm, we didn't have that g in phlegm when we borrowed that word or first used that word. But we dressed up medical terms, we dressed up va… - Arika Okrent

We were also spelling that oo sound as ou, because that's how they spell that sound in French, so it made sense that moon had that spelling but then the spelling didn't make sense anymore after the sound shifted, then it shifted again, but in some places it didn't

The vowel system was undergoing a big change, and we were just bringing English back into written form again after it had been basically out of written form for a couple hundred years

No one takes as many pages as English to explain all the exceptions, all the irregularities, and it's really the matching of the spelling to the pronunciation that causes a huge headache for learners of the language