Most seforim are Self-Published meaning the author pays all expense related to publishing the book . if the publisher had paid for the expenses alot less would be published.
Many other times like Feldheim will publish the book for free, but the author gets no money. I dont know about CIS or Art Scroll. I had a relative who had a book published by Feldheim and I was shocked they were not paid.
There are too many out there as the market is somewhat limited for them There are only so many people buying these books and there is only so many book these people want (How many inspirational books do most people have in their libraries)