My pet-hypothesis is that part of the problem isn't the tech per se, but that office workers do parts of jobs that would have been the specialty of several different people before, say, the 1980s. And not only that: they switch between those, and aspects of their own job, much faster than before.

A person who had two kinds of tasks to worry about in a given day in 1975, might have twenty today—technology didn't so much eliminate work, as allow it to be more concentrated. Someone's still doing the work, it's just five people instead of twenty-five, and none of them as is focused or specialized as before. Everyone's a secretary now, in other words—plus whatever else they do. Everyone is the mail room. Little bits of jobs like project management or plain ol' management get devolved down to ordinary workers. And so on.

Yes that's what "productivity gains" look like.