Well, according to iFixIT (Step 9 of the teardown), the memory chip is a Samsung KMRH60014A-B614
I am admittedly not an expert in this kind of matter. On the Samsung homepage of this chip I don’t find the specifications. But according to everything I do find when searching on the web, Samsung LPDDR3 chips are not available with 128GB. 64GB seems to be the maximum there.
So, a change for 128GB would mean a different chip with probably a different layout, even if the size does not change. That would make up for a technical reason, wouldn’t it?
And there is another point, I stumbled upon when searching for infos:
LP standing for Low Power seems to imply, that the chip is neccessarily soldered to the board. Chips that can be plugged in/unplugged are “simple” DDR, i.e. not Low Power.
That would mean a higher energy consumption and possibly a less long lasting battery.
And a socket for the possibility to change the memory chip would clearly take up more space.