I've always found it incongruous that we can move a person's body around the globe at 900 kilometers per hour for 25 hours for under $1500 but for 1000x that price and even with 1000x that time we can't bridge the distance between a brain and a foot.
A typical human cell is on the order of 10 micrometers. If you need to bridge even 1 cm of that, you're bridging ~a thousand cell-widths. If you think of a cell as the somatic equivalent of a house in a city, that's the equivalent of an infrastructure project spanning (based on a quick count of the number of houses on each block in Oakland) the equivalent of around six miles, or roughly from downtown Oakland to El Cerrito on a map of the Bay (~4 BART stations). And you have to do that on a scale where precision manufacturing is incredibly hard, where you're dealing with extremely difficult problems of chemical synthesis, in a living body, without provoking the body's defense or repair mechanisms to stop you. And that's assuming you even know what you're trying to do, which requires an understanding of the machinery of those cells that we often don't have.