Why is English spelling so inconsistent?

Three forces produced the mess. First, English borrowed enormously, from Old Norse, Norman French, Latin, Greek, and later everywhere else, and it frequently kept the source language's spelling conventions rather than adapting them, which is why we have ph for f in Greek borrowings and silent letters in French ones. Second, the printing press arrived in England in the late 1400s and began fixing spellings in place, sometimes on the choices of individual printers, some of them Dutch speakers, before any standard existed. Third, and most consequentially, the Great Vowel Shift moved the pronunciation of English long vowels dramatically between roughly 1400 and 1700, after printing had already begun locking spelling down, so the written forms record a pronunciation that stopped being accurate.

That mismatch explains specific oddities: the silent letters in knight, knee, and gnaw were once pronounced, the gh in through and though reflects a sound English lost, and the e on the end of name marked a vowel that had already gone quiet. Later scholars added insult by respelling words to display Latin etymology, inserting the b in debt and the s in island, neither of which was ever pronounced. Reform efforts have repeatedly failed, from Noah Webster's partially successful American simplifications, which is why Americans write color and center, to George Bernard Shaw's campaigning, because a global language with hundreds of millions of literate users and centuries of printed material has enormous inertia against changing its written form.