Because the developers actually want to make GOOD games. When movie makers want a game attached to their movie, the developers are deadlocked into what the movie maker wants...because of this the game isn't made well and ends up sucking.
Ok transformers 1 and 2 did have sam and mikeala but high moon decided to focus just on the robots since they made T DOTM and T WFC did you see them in those games. No. But in the 1st and 2nd they did have them soooooo. And in the 2nd in some point they have them in the game