There are certain words in the English language that people have problems with writing, but can pronounce just fine. Words like diarrhea. 

A common myth is also that there are only 5 vowels in the English language, namely A, E, I, O and U. This is obviously false, since we learn in phonetics class that there are 21 vowels. The 5 written vowels can be long and short. 

There are many complicated things, like homophones, which are words that are spelt differently but sound the same, words that are borrowed from other languages, and even more troubles arrive when you look at different accents.

In short, English spelling could be way worse when you would cater in all the ways you say things. Grammar would be so much harder to learn, and we wouldn't want that for anyone.