because the word is of French origin. The ending 'is' is pronounced /i:/ (no 's').

Because it's anglicized French. English borrows words from other languages. Sometimes it changes the spelling - not the pronunciation. Sometimes it changes the pronunciation - not the spelling. Sometimes it changes both the pronunciation and the spelling - but not necessarily so that the new pronunciation and new spelling "match." This is from that third group. The spelling is slightly different from the French spelling; the pronunciation is slightly different from the French pronunciation. But it has enough of the features of both the pronunciation and spelling that the word doesn't sound like it is spelled in English. In school, English kids spend a lot of time learning how to spell the words they hear and how to pronounce the words they read.