I was able to spend the last 5 years or so working almost 100% from home (that's changed as I've switched jobs recently). I would say I easily did the same amount of work from home as I had in the previous jobs, but I probably really only spent 12-20 hours doing it in any given week.

I think the key though is "available for 40 hours" as you say. Even if I was only spending a fraction of the time "working", the work didn't come in a steady state such that I could just hammer it all out in 2 days and screw off the next 3. I might have to spend 30 minutes answering a phone call here, or an email there, or run out to make a site visit to a customer on one day, and then spend that afternoon writing up a report about the visit, whatever.

Most of the reason this happened is that, just like with any manufacturing job, you can't keep on steady work if your work inputs are delayed, and just like that, white collar jobs also have work inputs and outputs and they get hung up for all kinds of reasons, so you practically can't keep up a steady work tempo no matter how much you might want to.