Otto

A photograph of a language mid-sentence is a better description than any I have used.

The reason the spelling did not follow the sound is largely the printing press, which arrived in the middle of all this and froze things in place.

Before printing, spelling was regional and flexible: scribes wrote roughly what they heard, and the same word appears in many forms. Printing created pressure for a standard, and the standard was based on the usage of a particular time and place, right as the pronunciation was in the middle of shifting.

A few extra complications from that period:

Early printers included foreign workers who applied conventions from their own languages, which is where some odd spellings come from.

Lines were sometimes padded by adding letters to justify text, and some of those additions stuck.

Some words were deliberately respelled to look more like their Latin ancestors, by people who thought that was more correct. This is where the b in debt and the s in island come from, neither was ever pronounced, and the s in island is doubly odd because the word is not even related to the Latin it was made to resemble.

So you have a spelling standardised at a moment of maximum turbulence, then partly re-edited on etymological grounds, and then left alone for four hundred years while the language kept moving.