Writers have to believe that what they write is worth reading. But since many are shy and very uncomfortable with self-promotion, they would prefer to have someone else do it, such as their publisher.
If your publisher is a small press, they usually do the best they can. But they just don’t have the resources.
And if you have a major publisher, they use most of their resources to promote known, popular authors or books expected to sell well. These only rarely coincide with literary works.
One can’t find readership if potential readers don’t even know you exist.