A lot of grammar is not arbitrary but just has a very rich history that is not apparent and sometimes not even traceable anymore. Schlüssel basically is a comb

A lot of grammar is not arbitrary but just has a very rich history that is not apparent and sometimes not even traceable anymore. Schlüssel basically is a combination of schließen (close) and the ending -el. Here this is the third meaning of -el listed on wiktionary: "suffix in agent and instrumental nouns". (https://en.wiktionary.org/wiki/-el#German https://en.wiktionary.org/wiki/-el#German). As far as I can see all of the words that have this version of the -el suffix are masculine. Languages are full of those rules that just are not apparent anymore, because they are so deeply layered and often apply to only a few words that they are not useful to learn.