Wish I'd stuck with English longer than I did, then I'd likely know an actual academical answer. As it is I can only make an educated guess that careful planning isn't a very big element of the natural development of communication, and also that anglophone speakers aren't much for phonetic spelling.
The Wikipedia article on English orthography seems to be a good read, though:
http://en.wikipedia.org/wiki/English_orthography
Also, there's an old phonetic parlor trick often attributed to George Bernard Shaw that consist of letting people guess how the word "ghoti" is pronounced.
"Fish." gh as in tough, o as in women, ti as in nation.