Why CGI giants? Why not use real actors and make them big?

I understand that in movies like King Kong, they used CGI cause it was an ape. I understand why in Jurrasic Park, they used CGI, cause they are dinosaurs. But in this movie, the giants are just giant men. So the CGI comes off looking rather fake, when they try to make mens faces from CGI.

Why not just use real actors? And I think of a few that would have made great evil giants.