In terms of how DRM is cost-effective for platforms and publishers (i.e. why they bother with it at all), it seems much more likely to me that it's primarily meant to prevent fully legitimate post-first-sale transfers of books, e.g. library lending, second-hand sales, and so on. Publishers couldn't do anything to stop post-first-sale transfer of physical copies, but DRM-enabled ebook platforms give them a new lever.
In terms of how DRM is cost-effective for platforms and publishers (i.e. why they bother with it at all), it seems much more likely to me that it's primarily meant to prevent fully legitimate post-first-sale transfers of books, e.g. library lending, second-hand sales, and so on. Publishers couldn't do anything to stop post-first-sale transfer of physical copies, but DRM-enabled ebook platforms give them a new lever.