Phonemics is more important than phonetics for these things. Sometimes two sounds need to be represented with the same letter if they are similar and their difference is context-dependent.
Then there's also the etymology and handling of grammatical endings. Polish spelling would be more difficult without "rz" for example, despite its two sounds already existing elsewhere in the spelling system.
"Context dependent" here means a different thing than it means in English, where the pronunciation of letters depends on the word they're in, without any hard rule. To the point of being pronounced in different ways even in homonyms: e.g. "tear" noun and "tear" verb.
The fact that there are a few rules on how to pronounce combinations of letters (and even a few exceptions here and there) has nothing to do with the total mess that is English.
Would it be sane to have a special letter to distinguish the "p" in "park" from the "p" in "spark"? In some languages, it's important, but these two sounds can be represented by the same letter in others because they don't "compete" for the same contexts.
Then there's also the etymology and handling of grammatical endings. Polish spelling would be more difficult without "rz" for example, despite its two sounds already existing elsewhere in the spelling system.