Why so many words in English are pronounced different from their spelling?

As a non native English speaker, I am astonished at the amount of English word pronounced different from their spelling.

For instance, 'Would' is pronounced like 'Wood', 'Whole' is pronounced like "Hole", and despite 'though', 'through', 'thorough' and 'thought' have similar spelling, their pronounciation is just vaguely similar, among many others.

I understand people pronounce words as they want regardless of spelling, but, is there a particular reason why this happens?

Thank you very much.