That works on computer systems because the PCI-E bus is already designed to be external. There's no need to separate it from the system bus because it's already separated in desktops.

In an ARM System on a Chip (SoC) like the Switch uses, the GPU is on the same chip as the main processor and very tightly integrated with the rest of the hardware. It's much more difficult to decouple this from the main unit. I don't doubt that USB 3.0 could handle the raw speed, but I doubt seriously it could keep up with the latency demanded by most applications.

An HD dock would almost necessitate an entire higher power SoC in the dock, and some way to synchronize the state between that and the one in the main system to pull off the system's characteristic fast-docking for which it's named. It's possible, but at that point it's less "offloading" and more "here's an actual high-power console that reads its data from the tablet."