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.