What? That's the exact opposite effect that happens with wide columns. With a wide column, your eyes have to scan all the way left to right. With a narrow column, you don't have to scan as much.
Also the largest "adult" books (non-textbooks) that I have take up about 1/5 of my 43" 4k monitors screen. If you aren't adding a healthy text margin, my eyes have to scan left to right twice as much as with a book. If you want to talk about novels - my screen is 8 times as wide as them, so they are naturally using much narrower columns than most websites.
Also the largest "adult" books (non-textbooks) that I have take up about 1/5 of my 43" 4k monitors screen. If you aren't adding a healthy text margin, my eyes have to scan left to right twice as much as with a book. If you want to talk about novels - my screen is 8 times as wide as them, so they are naturally using much narrower columns than most websites.