thing about transistors is that it's easy to have many transistors on the same IC behave in set ratios of each other (By changing the transistor width and lengt

thing about transistors is that it's easy to have many transistors on the same IC behave in set ratios of each other (By changing the transistor width and length ratio, etc), but getting a performance metric exactly controlled is fairly difficult (not saying it's not possible, just something you try to avoid because cost, reliability and yield). I'd imagine any hypothetical implementation of a NN will be fairly difficult because of that. Also achieving linearity cheaply (especially multiplication) is very difficult.