scenario scripting also adds heaps. and making decent cutscenes. realtime cutscenes generally require good programming, great animators, and a lot of time (among other things) and can add heaps to the development time of a project.

additionally, a good multiplayer mode usually (not always necessary though) has heaps of game modes which can require lots more programming, including the AI for those game modes (which can be much more than just an adaption depending on the game).

some games need very very level-specific code -- not just cutscenes and scenarios and whatnot, but actually have totally different gameplay for different levels.

and a game with heaps of different types of vehicles would only feature a couple in the demo, and those would tend to need individual programming, planning and design (such as hover-vehicles, flying, driving, balancing, whatever).

julz