I probably don’t know enough about computers, but wouldn’t there be a way to use e.g. the digital outputs of a DAC directly even in the earliest part of boot? Is there a good reason why CPUs aren’t doing this already?

Well, if you have one, and if the thing it's listening to is sufficiently random - both of which are in question here.

This is sort of the problem the CPU random number generator is intended to solve, but see the discussion on trust.