Orthography is more about disseminating communication in an asynchronous way than pronunciation. Over time, it crystallises into something most people can agree on and use to communicate; however, in that time, it will pick up relics that are hard to get rid of. It is also true that while it reaches a relatively stable point, it still evolves slowly (e.g., in texts from about 100 years ago, you sometimes see "cooperate" spelled "coöperate"; in really old texts, you'll see the "long s", like in "highneſs").
The main point is that writing is a human invention, while language itself is thought by most to be a natural cognitive facility, similar to face recognition, that we've evolved to survive and flourish. When a spelling was coined for a particular word, chances are that it was relatively close to its pronunciation at the time, but oral language changes faster than written tradition.
> but oral language changes faster than written tradition
Yes and no! Sure, if they evolve at different speed, oral changes faster than written. But, in order to have any difference at all, you have to have accepted them to diverge at a certain moment in the past.
Luckily :-) this was never the case in Italy. Starting at least with Romans, written has always been meant as a phonetic transcription of the language. The pronunciation rules did change during time, but the underlying correspondence principle has never been given up.
The main point is that writing is a human invention, while language itself is thought by most to be a natural cognitive facility, similar to face recognition, that we've evolved to survive and flourish. When a spelling was coined for a particular word, chances are that it was relatively close to its pronunciation at the time, but oral language changes faster than written tradition.