Its actually difficult to for a "regulator" to do what battery does, the battery can output significant surge as needed and its resistance drops as voltage rise

Its actually difficult to for a "regulator" to do what battery does, the battery can output significant surge as needed and its resistance drops as voltage rises absorbing large spikes without issue. In the Cybertruck there would need to be a DC-DC buck that could handle the 48v inrushes properly from the 800v traction battery and spikes would have no where to go as the DC-DC would not be bi-directional or even if it was could not react fast enough.