Publishers still often do some kind of promotion, e.g. ensuring that review copies get to influencers, hawking the book at book fairs, etc. Self-publishing mean

Publishers still often do some kind of promotion, e.g. ensuring that review copies get to influencers, hawking the book at book fairs, etc. Self-publishing means you have to do all the promotional work yourself. Also, certain books in the humanities can have their costs covered by grants, and the publisher might already have staff that are proficient in grant-writing, while authors don't have that savvy or reputation.