It's far cheaper and easier to just pluck a readily available 12V power supply off the shelf than it is to design one that will have limited applications outside of a single manufacturer.
Hobbyist computing could benefit from a move to 48V as well, if only to keep the problematic 12VHPWR from killing expensive video cards.
I say hobbyist because AIUI 48v is making inroads in server hardware but that's not my area.
You generally can't reuse non-automotive power supplies in automotive because the requirements are very different.