Because “psyllium” comes from Greek, and in Greek, the “p” is pronounced, along with the “s”. English has a tendency of simplifying consonant clusters found in other languages (like Greek) that we don’t have. For example, words like “mnemonic”, “pneumonia”, “pterodactyl”, “chthonic”, “gnostic”, etc. all come from Greek and in Greek, each letter would be pronounced, there’d be no “silent letters”, but in English, we change the pronunciation of these words to better match sounds we have in English, but maintain the same spelling.
Bottom line is: English words come from a variety of different sources and languages and English has a tendency to maintain the spelling of the word, but “Anglify” the pronunciation (which often includes things like reducing consonant clusters at the beginning of words, many of which are Greek in origin). Not to mention, English went through a major vowel shift where basic English words (like “hear”, “see”, “read”, etc.) underwent a shift in pronunciation, but the spelling remained the same, thus there is a disconnect between the spelling and the pronunciation of many English words. If any pronunciations change over time for whatever the reason, rarely do the spellings change along with them.