They take up valuable screen space, it is annoying to scroll to the sections you need. Yeah yeah some PDFs have the side navigation thing. Most don't
With a book I can put in those little flags to bookmark sections, I can easily riffle the pages and scan for the chapter I need, I can hand write in the margins
I often need 2 or 3 books open to different sections, I like keeping them on my desk so I can glance at them when I need to
I've probably cracked $1000 spent on books this year.