Though I haven't verified this at idle, just running the coolant pumps is actually pretty significant power draw as well. And they do run for things like Sentry etc. (probably because those compute modules are liquid-cooled). One of my (now apparently silly) notions getting into EVs was they'd be great for stop-and-go traffic because they don't "idle". Boy was I wrong - it'll consume 500W just sitting in traffic in gear (aforementioned pumps and computers, plus daytime running lights). 500W!
For the compute power draw, they probably just weren't designed for "clocking down" like a consumer computer. They were also never designed to be glorified dashcam setups, however they are now used as such. Had that been important original design criteria, things might be different. This is the downsides of adding features in updates vs. at design time -- sometimes the hardware around them isn't optimised for the task. But it sure does generate hype.
