CGI simply can't capture the full range of possibilties and even imperfections of reality, no matter how sophisticated the software or the abilities af the artists involved.
Not yet but I wouldn't bet on it never being possible. Superman Returns was made several CGI generations ago. Compare it to John Carter (these are movies that I've seen, there are probably better examples).
Also, it could be argued that many CGI characters are a mix of puppetry and CGI when you see Andy Serkis et al acting out the actions in real life with the spotty suit on.