Book publishing is a rather fragmented market and a lot of the growth is in self-publishing, which Amazon already own a big chunk of in various ways. I'm not sure why Amazon would have any interest in an old school publisher.
Looking at Publishers Weekly, I see this:
"An important driver of print book sales last year was the continuing increase in backlist sales, McLean said. Backlist titles accounted for 67% of all print units purchased in 2020, up from 63% the year before. In 2010, backlist accounted for only 54% of all unit sales. " https://www.publishersweekly.com/pw/by-topic/industry-news/b...
I think you could make a strong argument for buying a large publisher (or anything really) given a good ROI.