The problem with external gpus is that the pcie5x16 slot requires signal integrity chips every 5 inches or so [1]. Even for pcie4 it's bad, many people had blue screens or boot issues when using pcie3 only rated riser cables with a pcie4 video card, even though electrically they have the same connections. So, having a huge cable with x16 lanes of pcie5 on the back of the computer doesn't seem that feasible. Maybe fiber optics could be a thing.
Fiber optic would have higher latency vs. copper. Not sure how much of a difference that would make on a bus but I would assume the timing is pretty tight.
[1] https://www.planetanalog.com/signal-integrity-challenges-set...