The publisher is responsible for all aspects of publication. Publishers focus on authors they believe will have commercial success with writing, as publishing is a business focused on turning a profit.
Once your book is on the book production line there is a lot involved before the book is put in stores. They have to come up with a book design, then edit the book, typesetting and proofreading, cover brief and preparation of cover art, then program meetings, then a cover copy, followed by marketing and sales briefs, then a cover proof and printing, followed by text printing and binding, then the delivery to the warehouse and finally, the delivery to stores.