With nano-scale 3d Printing I am pretty sure we could make some kind of small robot

Yes but it is not like cells arent mechanical in nature. A cell is filled with molecular machines and with advanced synthetic biology it is possible to design the unevolvable.

For example cells can secrete proteins to nanostructure 3d inorganic substrates with unique properties, but nature only makes limited use of such. Humans can engineer novel proteins to create novel nanostructures for arbitrary purposes.