You may argue it's not a very good phonetic script, with all of the special cases present... But letters mostly represent sounds, unlike written languages where characters represent concepts.

A computer program written with knowledge of a few dozen of the common special cases would pronounce >90% of English words correctly and be close on most of the rest, "cupboard" notwithstanding. Indeed, look at how well 80's-era speech to text did with this exact approach.