To get 128GB of RAM on a GPU you'd need at least a 1024 bit bus. GDDR6x is 16Gbit 32 pins, so you'd need 64 GDDR6x chips, which good luck even trying to fit that around the GPU die since traces need to be the same length, and you want to keep them as short as possible. There's also a good chance you can't run a clamshell setup so you'd have to double the bus width to 2048 because 32 GDDR6x chips would kick off way too much heat to be cooled on the back of a GPU. Such a ridiculous setup would obviously be extremely expensive and would use way too much power.

A more sensible alternative would be going with HBM, except good luck getting any capacity for that since it's all being used for the extremely high margin data center GPUs. HBM is also extremely expensive both in terms of the cost of buying the chips and due to it's advanced packaging requirements.