It’s a Romanized tradition of transliteration. Essentially, because of the different alphabets of Latin and Greek, the Romans rendered Greek texts, names, places etc. as if they were Roman. This was in part down to the nature of the Latin script more or less being able to accurately render Greek with its own letters, and, more to the point, the fact that Latin at the time did not have as many Greek letters as we know today (I’m thinking specifically “K”, where a lot of Roman readings of Greek use a “C”). This extends also to letters in Greek represented by digraph (Χ = Ch, Φ = Ph etc.)

Why did this stay? Well, Western Europe received the wisdom of these Greek writers through the Romans mostly in Latin, and therefore the spellings stuck. Incidentally that has led to their pronunciation changing quite radically along with the pronunciation of Latin letters in non-Latin languages, but that’s a whole other story.