Because even a reasonable set of first steps towards creating artificial mechanical life forms made out of arbitrary atoms do not exist and probably never will. Drexler wrote a stupid science fiction book. Good marketing for chemists for a while I guess. Remember the congressional hearings back in 2005 on how nanotech was going to be 15% of US GDP by 2015? I do!
https://www.govinfo.gov/content/pkg/CHRG-109hhrg21950/html/C...