No they don't. I have a Dell Optiplex 3070 SFF (not even ultra small form factor, it is standard desktop sized) that idles at 8W. It has an i5-9500, 8GB memory, and a 128GB NVMe.
Various Orange Pi boards idle at 1-1.5W. The latest ones around 2.4W (not optimized, yet).
My Rpi Zero idles at 0.2W - it's nor in the same computing class as SFF/NUCs. I don't run a homelab in it; and I find it ideal for ambient, always on compute/networking/monitoring tasks
That's not totally true. I have a Dell Optiplex mini-PC version (and not that new) and it draws on average 10W, which is surely more than a RPi4 but not 10 times.
Farily comparable RK3399 idles at 1.1W.
Depends, there are Celeron N systems in the <2W idle territory thanks to the smaller node process.
As a reminder, Pi 5 no longer recommends to use the 15W supply recommended for Pi 4 and suggests a 25W one instead.
25W PSU required but people complain about a 10W SFF PC.
Schrodinger's power consumption.
The n100 I have is 6W. It uses less power than my OrangePi board.
12V barrel connector.