Why does "carpenter" end in "er" if "carpent" isn't a word? Same with "butcher". (As in: "I butch for a living.")
Because of their etymology.
ORIGIN Middle English: from Anglo-Norman French, from Old French carpentier, charpentier, from late Latin carpentarius (artifex) ‘carriage (maker),’ from carpentum ‘wagon,’ of Gaulish origin; related to car.
ORIGIN Middle English: from an Anglo-Norman French variant of Old French bochier, from boc ‘he-goat,’ probably of the same ultimate origin as buck.