If you have a normal size backpack carrying a normal size load like you are going to work or school, it’s easier to slide in a smaller paperback than a bulkier hardcover. The extra volume makes a difference in this case.
I can think of one time in my adult life that my backpack has been anywhere close to that full, which was a weekend trip to New York City where I was testing whether I could get away with not needing to navigate the transit system with a large bag (it worked but didn't really feel worth the effort). A hardcover would be no issue to fit in any normal situation.
That said, when I travel I carry an e-reader and usually pick up a car magazine at the airport newsstand. My physical books stay at home.
“Can’t fit a hardback book into a backpack” is nonsense. Even in middle school and high school hardback books are often exactly what is filling the backpack so full.