A big part of that is that the whole of the game is built in a rush by people with no free time who are probably chronically sleep deprived. The machinery of the game ends up being essentially propped up on sticks and held together with bailing wire. As such the stories that you can fit in such rickety and uncertain machinery is typically the most linear. The more complex and dynamic a story is the harder it is to avoid big bugs within the story itself or due to the interaction of the story and the game engine. It also makes testing much more difficult, which is already short on resources for most games.

There's so much churn in the basic technology of gaming systems that it makes it difficult if not impossible to concentrate on only story telling.