Because spoken language has a couple thousand sounds yet we only have 26 letters. If you want to keep the language an alphabet, it'd be impossible to not have letters affect the pronunciation of other letters. Not that English couldn't do with a spelling reform but that could never get rid of all different pronunciations for the same chains of letters.Just spell slaughter as slawter.
There's a lot of reasons why English spelling is bad.
Pronunciations change, and the spellings don't. For example the words knife and knight, once upon a time you'd pronounce the 'k'.
We also borrowed a lot of words from other languages like French especially, Greek, and others without properly changing it.
Japanese doesn't have that issue, but it largely helps they have fewer sounds as well as a strictly phonetic character group.