Etymology is the reason there are so many silent letters in English spelling. Etymology is the study of the history of words, and there was a widespread view that words should show their history in the way they are spelled. There was a genuine belief that it would help people if they could ‘see’ the original Latin in a latin-derived English word.
So someone added a ‘b’ to the word spelled ‘det’, ‘dett’, or ‘dette’ in Middle English because the source in Latin was ‘debitum’. It thus became debt and caught on. Similarly, an ‘o’ was added to ‘peple’, because it came from’populum’. We find both ‘poeple’ and ‘people’, before the latter became the norm. An ‘s’ was addedto ‘ile’ and ‘iland’ because of the Latin ‘insula’. Now we have ‘island’. There are many more such cases. Some people nowadays find it hard to understand why there are so many ‘silent letters’ of this kind in English. It is because other people thought they were helping.