Because it's not supposed to be there.Holy Putting lib_nx.bin straight into romfs really does work! I finally managed to increase resolution, however it also broke many graphical effects for some reason. Will experiment I guess now that it finally works.
I am not sure if this lib_nx.bin is even read there, I guess it's setting default settings that is inside executable instead from this file. But for one of my testers lack of this file resulted in crash instead of fallback to default values like in XC2, so I was expecting crash also from other people