Good question. Like many that have anything to do with English orthography, the answer is not a simple one. The word came into Middle English from Norman French sometime after 1066. It was spelled enditen, and the vowels were all sounded as in Italian or Spanish. Somewhere between 1066 and the 1500s, there was a movement in Parisian French towards spellings that indicated the Latin etymology of words. That's were the silent b in debt comes from, (cf. the too-often pronounced t in often). Sometimes they got the etymology incorrect, as in admiral, sometimes correct as in perfect (originally borrowed as parfit) or adventure (aventure).

Ceci n'est pas un seing.