They also tend to get done very quickly. Or aren't actually worth doing.
I've yet to find a job, as an IC or manager, where I had 8 hours worth of tasks worth completing.
Oh, there's usually -work- enough for 8 hours, every day, if I want, but it's actually a negative effect to try and do so. It means I'm getting involved in decisions I don't have all the context on, am not responsible for, and am not empowered to actually change (and thus just getting frustrated and probably pissing someone off), or I'm trying to work on something that requires buy in from other people, that hasn't been prioritized for them, so is going to die on the vine anyway (and, again, frustrate me and piss them off).