It has been covered, but searching the fora doesn't work too efficiently. (Maybe there has been a post about the difficulty of searching the fora, but I didn't find it?)
The basic answer is that the WGT business model is based on a strong commitment to the cloud computing concept, with small continuing fees charged for players who want to play the game at a competitive level (even though technically speaking you can play the game for free). The game is thus web-based and everything is on their servers. Besides the business aspects, the amount of information for any course, with photo-based high definition, is vast, so probably the offline mode is not feasible anyway.