As a counterpoint (to a degree): why 12V?
All the 'high power' components that 'run' off of the 12V rail don't. Instead, CPUs, GPUs etc use 12VDC as merely an input for their own DC-DC stepdown conversion to the votlage they actually need, usually ~1V.
Now, distributing 1V inside the case is not a workable idea. You'd need bug chunky conductors, and modern CPUs/GPUs need rapid self-regulation due to the way they vary their operating voltage dynamically, so you would not even eliminate local converters.
Instead, raising the internal distribution voltage makes sense. 24V or even 48V are good choices as there are plenty of discrete components already available to deal with these voltages. You could use an arbitrary voltage like 50V, but then all your discretes need to be custom or wastefully overspecced. Higher voltages allow for more efficient DC-DC conversion, and also allow for finer gauge internal wiring for the same power capacity, due to the reduced current. Potentially they could allow for high power distribution via the same connectors that carry bus interfaces (e.g. increase the power cap for PCIe slot power, up to ~250W for 48V, without increasing the current limit).
The drawback is the lack of backward compatibility. Not a problem for motherboards, as you've already eliminated 3.3V, 5V, etc, anyway, but add-in cards would need to be distinct between 12V versions and 'high volt' versions, and/or replace their on-board power conversion hardware with a 'flexible' design that can handle a large DC range, or add an extra stepdown stage before the normal stepdown & regulation stage.
