> My first question would be, why has no industry trade group attempted to define a standard socket, or use an existing physical socket pin-out.

Because ARM is a design licensed to companies that want to make their own chips for their own purposes. Those purposes involve optimizing cost by choosing, per product, whether to integrate various cores onto the SoC, or to leave them as external devices on the board, or to maybe put them into a secondary chipset microcontroller and route some IO pins to connect the two.

In ARM designs, the licensee determines the required pin-out, because the licensee determines what the CPU does vs. what the board does. You can't really have a generic "ARM board", because no two "ARM CPUs" would have the same expectations for what are on that board.

A particular ARM licensee could standardize the socket between their own designs—but in doing so, they'd lose a lot of the advantage of licensing vs. buying an off-the-shelf chip in the first place.