Mainly, there are two reasons that stand out...

1) In the past, people made the spellings like that, so if you would have to change the spelling after writing it like that for over 10 years, it would definitely become hard.
2) In different regions, though with the same spelling, the same word is pronounced in different ways. So when you visit a foreign place, you'll get confused if you've changed the spelling.
1 5 1
In English never trust words to be phonetic.English language own its vocabulary to linguistic sources like: anglo saxon(Old English), Old French, Latin, Norse etc..... for example debt in this word 'b' is silent its because the 'b' sound got lost due to the French phonological rules. So, maybe the letters in words like "debt" where 'b' is silent because of certain rules there can't be a common reason.