Laptops are still pretty bulky and power hungry in comparison if you're looking for very SFF and passive cooling.
Pop open a cheap Celeron N laptop, and often, the motherboard inside is essentially a passively cooled SBC.
https://www.insidemylaptop.com/wp-content/uploads/2018/03/Le...
The big pain with using something like this would mostly be the IO and odd form factor.
You can passively cool a laptop CPU too if that's your jam. Plus, latest RPI 5 cases now feature fans to not thermal throttle :))