answer:It’s not. Cells just lack a brain, and so they don’t realize it’s impossible, and they go no and do it. Actually, the machine DOES have access to parts outside itself. Because it is so small, it is made of incredibly tiny parts such as hydrogen, carbon, oxygen, nitrogen and other atoms. It has access to a nearly unending supply of these externally. To make a copy of itself, it just needs to take these in, and then use chemical processes to turn them into long chain molecules, which it does all the time as a part of it’s job as a machine. I agree it’s mind boggling. That is why, I submit, it’s done only by things too little to posses a mind.