Chapters are released per week in Jump magazines, but they are meant to be read in volumes (they're meant to be read in clumps of 7-9 chapters). Not only that, but many people like to own a hard copy of manga, which is why they are also sold in book form.
it's just cheaper and more efficient. mass producing a 20 page chapter every week isn't really a good idea and it's better to do it with a 100+ page books every once in a while. the covers are just to attract attention and have something nice on there, like how books have covers even though they don't add very little to the book.
also it's a chance to fix errors, which is always nice.