A mix of historical change and language attitudes. English spelling was mostly standardised just before a [major series of sound changes](https://en.wikipedia.org/wiki/Great_Vowel_Shift)happened, and the spelling mostly reflects the pronunciation from before those changes. Spanish hasn’t had really much of anything quite so disruptive happen – it’s been more a long series of much smaller changes. On the attitude side of things, English speakers have made a huge deal out of the concept of ‘spelling things right’, to the point that major change is largely unthinkable at this point – too many people have too strong of feelings about the current spelling system. (This might also be due in part to English’s more major sound changes! It would take a massive reform to update English spelling, and it would have even if the reform had happened in 1600, thanks to the above-mentioned Great Vowel Shift – updating to account for even just that change would require a major change. Spanish on the other hand has largely been able to get by on a rolling series of small tweaks.)
Plus, now English has different standard dialects in different places, and it would be impossible to achieve a Spanish-like level of one(ish)-to-one(ish) letter-to-sound correspondences in all dialects simultaneously without having different spellings per dialect.
For some other examples, compare Tibetan – which has a worse spelling-to-pronunciation correspondence than English does – and Swedish and Norwegian, where Swedish has much less predictable spelling than Norwegian despite them being basically dialects of the same language (from a purely linguistic perspective). Norwegian has gone through a series of language reforms (not confined only to spelling) since Norway’s independence from Denmark in 1814, in part as a way of asserting a separate linguistic identity from Danish; Swedish just hasn’t ever had the same impetus to change. Tibetan went through a drastic change somewhat like English did, where several kinds of previous consonant distinctions got turned into tone distinctions all in one go; I suspect that’s also part of why Tibetan hasn’t been updated.
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