Why is English spelling so inconsistent?

Because English spelling was frozen in print at exactly the moment its pronunciation was changing fastest, and because the language has absorbed vocabulary from a dozen sources without regularising any of it.

Four forces did most of the damage.

The Great Vowel Shift. Between roughly 1400 and 1700, the pronunciation of English long vowels moved dramatically. Name had once rhymed with the modern calm; bite sounded closer to beet. The spellings, however, had largely settled before the shift finished, so they record how words used to sound.

Printing. William Caxton introduced the printing press to England in 1476, fixing spellings in mass-produced text well before any pronunciation stabilised. Some early printers were Flemish and imported conventions from Dutch — the gh in ghost is generally attributed to this.

Borrowing. English took ballet and rendezvous from French, psychology from Greek, yacht from Dutch, lieutenant from French via Norman, each keeping something of its original spelling. Few languages borrow as freely, and fewer still leave the spellings intact.

Deliberate meddling. Sixteenth-century scholars added silent letters to make words look more classical. The b in debt and doubt was inserted to signal Latin debitum and dubitare; it was never pronounced in English. The s in island was added by false analogy with isle, although the word is native and unrelated.

Silent letters are therefore often fossils — the k in knight and the gh in night were both pronounced once.

Spelling reform has been proposed repeatedly, by Noah Webster and George Bernard Shaw among others, and has never taken hold beyond the modest American simplifications.