It might help to remember that a human body has more cells than there are people on Earth by around three orders of magnitude, and that you're engineering things on the scale of nano- or micrometers.

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.