(I'm going to recycle one of my recent comments here: sorry for the rehash, but it's an important issue and one which is much misunderstood.)
Following my post Questions About Self-Publishing, a reader commented, "Another big myth is if an author sells his/her book to a publisher then the publisher will do all the promotion. This rarely happens."
He was almost right, but not quite. And that "not quite" makes a big difference.
Publishers don't allocate over-large promotional budgets to every single book that they publish; nor do they arrange book launches or signing tours for everyone. However, they do include every title that they publish in their catalogue, and it's this catalogue that their distributors' sales teams use as a sales tool when they visit bookshops.
Those sales teams visit every single bookshop in the country, as often as they can, to discuss new releases and hot titles. This is what gets books into bookshops; and consequently, books into people's homes.
Publishers also routinely send out advance review copies of every single title that they publish, to every appropriate publication that they can find, and to carefully-selected bookshops all over the country. Depending on the title they might send out 50 ARCs, or perhaps a few hundred: but this is something that very few self-publishers can afford to do, or have the mailing list and contacts to do effectively. And it's this that generates the most effective reviews, and gets booksellers motivated to promote certain books.
Even though publishers don't heavily promote every title, they are very effective at selling books. While it's true that publishers don't automatically spend big money promoting every single book that they publish, they do an awful lot to get them into bookshops: because bookshops are where most people buy their books, and where most books are sold.