With white-collar jobs and even most jobs, you rely on other people and there is always going to be a situation where the person that is needed is not there in person. So people not going to fill 100% of their time with 100% productive time because you have to wait sometimes. If you expect employees to fill 100% productive time that is unrealistic.

"WFH employees often describe themselves as you seem to do as an assembly line worker. If you want to do that, get an hourly job doing that."

Not sure I agree with this conclusion. If you have tasks to do in a day and are hourly you are going to fill every hour with work to get paid and NOT to do less. Also, I don't know where the "you build your own work" comes from, and here lies the other problem with most of the conversations in this thread, every job is different and we try to apply our understanding of "our work" to others. I honestly don't know how others fill their day with work and what does it look like. I know what that looks like for me. That is a client comes to me with a problem and I fix that problem so no I am not technically building my own work.