I believe my History of the English Language teacher would explain it this way. When English was adopting Latin and Greek words into the vocabulary, due to the lack of an equivalent form, the English sound system was well established. Certain sound combinations just didn't fit the pattern already in use.
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MUSCOVITEHow come the leading letter "p" is silent in the above listed "Anglo-Greek" words?Not to mention 'pterodactyl', 'ptarmigan', and 'pneumonia'. And, from German, surnames like "Pfeifer", proving it's not just prejudice against the Greeks.
It seems that there are actually many words in English with this beginning. However most of these words are not commonly used ones. I did a quick search and found this list http://www.unscramblerer.com/prefix-list/words-that-start-with/ps of 274 words starting with PS.
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