Not only does creating full 18 round courses have to do with signing agreements and getting the rights to use them.. it also has to do with setting up times to go out and literally take a ton of pictures of the course at multiple angles..
the way this game is created is by someone going out to the physical course and taking digital stills of each hole from different angles.. those images are then combined with actionscript and assigned parameters and variables and coded out to make up for all the different aspects and possible conditions..
..so as to your original question.. getting the CTTH courses avaiable in full 18 hole play would take wgt going back out to that course.. and taking entire new sets of stills and coding out the course all over which could take weeks if not months (and if im not mistaken i read somewhere while trying to find tips on improving my game that they do not plan on doing this)..
..as far as new courses go.. it can take weeks for them to go through this process.. and thats after the weeks it takes to get through all the red tape... therefore you just have to be patient and wait it out.. this isnt like other golf simluation/games where its a simply vector designer that can just take sample images and created a wired 3d layout of the course that resembles it naturally and add color...