Novels are alredy categorised by story type… sometimes. Othertimes, they are categorised by setting. And still other times, by target audience. And then there are those further extraneous categorisations (literary, reading group, etc).
And every book fits somewhere – probably manywheres – in each of those categorisation axes.
And becuse there are (so many) people who want to reduce everything down to a my-preferred-bucket boolean, certain spots within each categorisation get named genres, and never may they overlap (from a marketing perspective).