The speed of light works against you in that case. Even at the incredible 14Thz line rate achieved here, 1km of fibre can have ~146kbits bits in flight at a time.
Yes, though in this instance they pulled of 44terrabits rate per second, so with that you would be looking at 156 megabits inflight over a 1km cable if my napkin maths are correct. Thats 18 megabytes, sure error rate etc and correction would limit that a bit as nothing works as ideal, but sure does seem like may have some uses come into play.
46 megabits at 3.34µs per kilometer[1], however error correction brings that way down. Regardless 1KM of fibre is quite substantial, we're talking densities far lower than even 70s technology. Worse if you try to up the cable length for larger storage latency will increase with it.
Especially considering how cheap fibers are in comparison to patterned silicon. If the access patterns fit, it might be useful as high-bandwidth memory.