It's not an engineering problem, it's a political problem. If you present this problem to any power engineer at Nvidia, they'd probably say something akin to "yeah, delivering 600W at 12V over a 12-prong connector is insane, up the voltage". The issue is that 12V has been the standard voltage for ages, and if you want to sell a product that requires a higher voltage, you first need to get the industry to produce PSUs that deliver a higher voltage.