Some of us have been waiting for external GPUs for nearly a decade. I'm not sure a critical mass of demand exists for this type of interface.

Value gamers stick to mainstream ATX PCs, and often do their own work, as components can be much cheaper and the labor is free (supplemented by a few internet build guides).

High-end gamers often do their own work so they can ensure quality throughout the build and verify every component. Those that don't usually only buy completed systems, and replace these very frequently.

Almost all use cases where an external card device would be useful are already served by having a desktop instead of a laptop, where the discrete GPU is almost external, an ever-increasing stack of chips, fans, and radiators sticking out of a motherboard slot.