Y'know what's even harder to design? Analog computing. Holy cow. Remember, digital computing was invented by Touring before we even had built a computer. It's easy to visualize how it works. My brain explodes though trying to imagine a fuzzy-logic analog equivalent of a touring machine.
I used to think that AI research combined with neuroscience would figure out a simple solution to this problem, but it's increasingly seeming like, no, it's even complicated in the brain.
So people can pine for analog memristor computation, and analog optical computing all they want, but the hardware is the easy part here. Get the software side solved, and if you build it they will come. But it's not because we aren't used to these problems, it's because these problems are really really hard.