There just aren’t that many cheap paperbacks around any more. The entire publishing market evolved from the early days, when most paperbacks were indeed cheap, although Pocket Books normally used good quality paper - and dyed their edges - and serious nonfiction houses like Mentor also did. By the 1970s a funny thing happened: paperbacks, which once were so page conscious that books were known to be stopped in the middle of a sentence if they hit their limit, started becoming fatter and fatter. That required better paper quality, since that quantity of thick cheap paper was physically impossible to handle by the binders. In addition, most hardbacks, including nonfiction, saw paperback editions. There was a swing away from paperback originals, since planning from the beginning to print both editions made more money. By the 90s, though, trade paperbacks became the preferred format over mass market paperbacks, so mostly only genre books (and bestseller is a genre) got paperbacks, while other books either got a trade paperback edition or had their original publication in trade pb.
This may be an American thing. The old Penguins I have show plain edges, but then they were famous for color-coding their covers and I don’t know if that served any of the same functions. But it’s a very old American thing. I’d say it’s probably been 50 years since any company did this regularly. And cheap pulp paper is a thing of the past as well. Paperbacks are sold for today, not tomorrow, and they are made to the standards of what moves them off the shelf.
Shelf! That’s another thing. Paperbacks were once almost universally sold in wire racks, either flat racks that had up to a half dozen books in a partition sticking out from the backing or circular racks that were movable to see all sides. In either case, the edges of the books were visible - totally unlike today, except perhaps in an airport. I wonder if color-coding the edges allowed canny buyers to go straight to publishers to search for favorite authors. Weak, but possible.