In pretty much all modern computers and mobile devices, the CPU can have varying amounts of memory (either to be configured by the user or fixed at point of assembly). Apart from historical form-factor reasons (where GPUs just came as 'a card' that was actually composed of the GPU itself, power management, I/O and memory), is there any fundamental reason (ie, electronic engineering reason, not marketing) why GPUs don't come in different amounts of memories for a specific GPU model?
Of course, different models of GPU might have different amounts of memory, but to my knowledge, a specific GPU model will always come with a specific amount memory (for example, the AMD R9 280x in my workstation always has 3 GB of memory).
For general consumer applications, I can understand that there might be a cost/complexity argument - it is simply easier and cheaper to have one configuration. But for applications that are more GPGPU oriented, it seems like linking memory configuration to processor configuration could be limiting? (for example, my place of work recently acquired a number of GPGPU servers. Each of the GPU's comes with I think 24 GB or RAM, but the FDTD simulations they run don't need anywhere near that amount of RAM).