Some languages are completely phonetic, Spanish has a system where each letter has a specific sound and even the stress has rules, so you can see it written and know exactly how it's said (although depending on the regional dialect some sounds will be dropped)
English is a bit of an awkward, hybrid language. It's mostly a germanic language, but because of French influence (they ruled England for several hundred years), it is also approximately forty percent romance. As such, graphemes and certain spelling patterns were "borrowed" from other languages.