Power Supply (?) Unit (where you plug in power cord for your desktop, e.g.). Something that is enough for majority of the GPU cards ends up bulking up the size comparing to anything middle size/weight.
IMHO middleweight option is already about the size of a PSU, meaning that anything with PSU would already double the size and weight, putting it to heavyweight category?
Additionally if the card needs to be housed inside a case, the volume of the housing is dictated by rather the biggest GPUs out there than the smallest (You’re surrounding the space needed by the card by a case).
Easily ending up with a bulky box that doesnt resonate to people who just wanted a dock and prohibitively big to most who want it to be mobile.
Take both PSU and the enclosure out, though, and the dock and still support a GPU (it just needs PSU, and the card will be visible)