The reason is that it has always been assumed that there is a substantial body of wage slaves who will do it simply because otherwise they'll starve. However, if human intelligence is turned to the question of how to make the necessary work of the society itself meaningful, we don't know what the answer will be. My guess is that a fair amount of it can be made entirely tolerable.
It's a mistake to think that even back-breaking physical labor is necessarily onerous. Many people, myself included, do it for relaxation. Well, recently, for example, I got it into my head to plant thirty-four trees in a meadow behind the house, on the State Conservation Commission, which means I had to dig thirty-four holes in the sand. You know, for me, and what I do with my time mostly, that's pretty hard work, but I have to admit I enjoyed it. I wouldn't have enjoyed it if I'd had work norms, if I'd had an overseer, and if I'd been ordered to do it at a certain moment, and so on. On the other hand, if it's a task taken on just out of interest, fine, that can be done. And that's without any technology, without any thought given to how to design the work, and so on.