It's not just ethics. Biology is just plain complicated. It's a science which tries to reverse engineer the results of billions of years of natural selection.

It's not just ethics. Biology is just plain complicated. It's a science which tries to reverse engineer the results of billions of years of natural selection. A human body belonging to a lucky person might live the better part of a century, all the time which it grows, heals itself when injured, processes food into new resources, hosts an example of the best known thinking machine in the world (the human brain), and literally has the machinery to produce one or more additional human beings. These are capabilities which are incredibly difficult to replicate artificially, so much so that producing a robot that could do even half of these things remains firmly in the realm of science fiction. And it wasn't designed, it was evolved, so it's about the worst mess of spaghetti code you could imagine. It's simply a hard problem.