Because they can sell the "expensive" option (which, for the OEM itself, isn't even too expensive) at a much higher premium.
> I would also love similar initiatives in the mobile space, but I reckon it is more challenging.
In the mobile space it would be an even bigger exercise in futility: There is no, and will never be, a baseband chip with a free firmware. The FCC made that pretty clear back in the OpenMoko days – use our NSA-approved proprietary blob or you'll never sell in the developed world.