Phonetic spelling is useless when you’ve got more than one dialect. For instance, consider the word ‘castle’. Is this [kæsɫ̩], or [kɑːsl̩], or [kɑːsu] (as it is for me), or something else entirely? Having an orthography which is not purely phonetic avoids this problem.

Terry Pratchett plays a lot with language, and it is funny

> “Mr Vimes," said Mrs Winkings, "ve cannot help but notice that you still haf not employed any of our members in the Vatch..."

> Say 'Watch', why don't you? Vimes thought. I know you can. Let the twenty-third letter of the alphabet enter your life.”

I am not a native English speaker, yet I decifer everyone execept gargoyles:

— e cuns uk ere um-imes an awks oo ugg

Huh, what a coincidence — I just re-read Thud! a couple of weeks ago! An excellent illustration of my point that phonetic spelling is highly dialect-dependent.

(Oh, and don’t worry; I’m a native English speaker, and I can’t understand Pratchett’s gargoyles either.)

There are phonetic languages. For example, Hindi and probably most other Indian languages are written phonetically. What you write is what you speak.

Not quite. No script, even the IPA, is perfectly phonetic. Natural languages deviate in so many ways over time that this is the only possible eventuality.

Language is unconscious and you won't understand how it works until you let go of a lot of false culturally-programmed notions, and then actually study it by engaging with the work of linguists.

https://en.wikipedia.org/wiki/Coarticulation

For Hindi there are additional phenomena that will alter the sounds of spoken language in certain contexts.

https://en.wikipedia.org/wiki/Sandhi

Regional accents won't play nice with that.

Not in English, they won't.

In French, Italian, or German, though, they already do. Those languages are quite orthographically regular.