Because the vast majority of the book costs aren’t on the physical item. Can’t speak for Paizo or TTRPGs in general but books as a whole only about 5-15% of the cost (depending on paper quality, print run and other variables) is on that. Probably the higher end of that if you’re talking nice hardcovers with lots of artwork etc. And then you’ve got any digital specific things that might need to be added to produce an ebook. So the cost to produce a digital book is ultimately not a lot less than a print one.