First you learn how to name the things you see and speak their names. Then you learn how to write (and read) those names.

Think about a leaf you just saw. The leaf is on the tree, the sound "leef" is in your mouth and the letters "leaf" are on paper or a screen. They are all clearly very different. These differences have to be taught.

Consider that the English speakers could have chosen to write leaf in a different way. For example "leeph" or "lief". So every English sound can be written in different ways.

Consider the words written as "tough" and "rough". You pronounce "ough" very differently in those two words. So written English letters can have different sounds when spoken.

Then consider how different a word can sound when spoken by people from different areas. The written language is still the same, and represents those very different sounding dialects.

To conclude, you can't know how a word is written from how it sounds, and you can't know how a word is spoken from how it is written. You have to learn both, and they are not the same.