Theres a reason that accountants of old are characterised as sullen and miserable people, and its not because they were poor. Happiness for the most part is resiliant to physical hardship - happiness indexes among countries are almost completely unrelated to GDP.

Working this kind of mental & menial work clearly is missing something important that I'm confident we will one day understand a lot better. Perhaps its simply relative position (keeping up with the Joneses), or maybe its a need for much more movement (it's well understood that zoo animals need to be given very large areas in order to be happy and yet we often sit in spaces that would be illegal to keep animals our size in for the majority of our days)

Maybe it's access to nature itself - I often feel a lot better after taking some time to walk alone in nature, but perhaps this is because of something else than just simply nature itself. On the days where I remember to take a moment to step outside of the office, or just take a break from routine and do something slow-paced for the sake of it, such as taking a detour on the way to somewhere just for the sake of seeing what that way looks like, I find that I feel much happier for the rest of the day. However, this lacks an explanation for why I don't do this often and feel good all the time. My concious brain knows it is a good thing, but my concious brain is rarely in charge.

Maybe it's just that people need tasks on a different scale to those we work on nowadays. This I think hits a lot of people. I used to work at a campsite for a while, where I would spend the day cutting down a tree, or moving a pile of gravel. Whatever I was doing, it would take at most a couple days, and it was usually either just me or me and a couple guys around me. This diversity of tasks and short time to completion really made me feel much better. I also often think that removing tasks such as washing the dishes or repairing clothes meant that we overspecialise, which is removing even more of these such short and easy wins.

Perhaps its also the need to feel useful in a profound way. I often think about how before the internet it was possible to gain a reputation locally for being good at someting. How had the guy who fixed clothes, another who was good at tying ropes, etc. Often these skills would be unrelated to work, and I believe would have an important role in having a self-identity. Nowadays we naturally get fit into a single stereotype by our family and friends based on our line of work, but at work everybody is that way, and so a conflict with our idea of self arises that makes us feel worthless even if we know the work we are doing is useful. Perhaps somehow, we need to feel that we aren't replaceable.

I also think that there is an aspect of physical work just being less tiring, or at least in a different way. When I used to work a manual job, I would feel exhausted after the day, but in a way that meant that the next day I would wake up without feeling drained. I had no trouble getting out of bed or finding the energy to cook, clean etc. The tiredness was in my aching muscles, but it didn't affect my decision making nearly as profoundly. It's nothing like the brain fog that causes me now to come home and need the sensation of something forgettable and mindless like youtube or video games.