Well, that is true. But presumably selection pressure —- namely, the need for things to work reliably and in an organized fashion —- imposes structure on systems that parallels the kind of organization that engineers use to make complicated systems work reliably. There’s a reason that complex organisms concentrate specific functions into organs with recognizable interfaces rather than scattering those cells widely throughout the body, in the same way that a human-engineered mechanism is usually constructed from parts. The fact that this allows for organ transplants isn’t really by design, but it’s a convenient outcome.