Many people wouldn't get far into the great works of literature with this approach. Many worthwhile books are "difficult pleasures" but the pleasure might not come until after experiencing a lot of pain. Things that are worth doing aren't always fun but you get through the displeasure to become a different person -- one who has access to pleasures you may not have realized you could experience. I didn't like beer when I first tried it. I kept trying it and now I know why beer is beloved by so many.
Many worthwhile books are difficult, true, but many are not. You'll never read all of either group. This advice is saying: don't waste time on books you're not getting anything out of, if it means missing out on books you would get something out of.
Honestly, you could read all the greatest books several times in a life with a habit of reading and a good prioritization. Many great books are pleasurable but many pleasures are acquired. The pleasures of middle age are not the pleasures of youth. The things you might toss aside one year are the things you cherish a decade later. If a reader doesn't get something out of a great book then that's a problem with the reader more than the book.
