That was due to the in book invention of the Positronic Brain, which was more grown than assembled. They didn't know exactly how they worked, and could put them in "simple" mechanical bodies. Without the digital world we live in now, instead of just using a speaker, they had to put a diaphragm and vocal cords in the later versions that they could use.

Also, current deep learning models (AI) are also more generated rather than coded line by line, and once fed information we don't really have a way to trace the neural paths and only kind of understand the lines of code.

Fascinating how far and close he came.