Combat is the main activity in most games because combat is tried, tested, simple and easy to implement. Developers and players alike are used to it and know what to expect. It's safe. Of course you can have lots of different weapons, spells or whatever, but however complicated your combat system is, the basic type of encounter is usually a fight to the death, the standard action an attack. You may also evade or defend, but basically you attack. Typically, game worlds are filled with hostiles who attack the player on sight, unless the player attacks first.
A testament to this predicament is that whenever developers try to implement something else, the result is likely some awful minigame or irritating quick time event.
At least you sometimes have elaborate systems around the central theme of conflict and violence. A popular activity is collecting herbs and ingredients for potions, for instance. In games like Skyrim or Two Worlds this activity is entirely optional. But I can imagine a game centered on it. Instead of the usual combat super hero, you would play a weak alchemist, somebody who would stand no chance at all in a fight without powerful potions. There would still be situations of conflict and violence, but the main game would consist of exploration and research, gathering knowledge and ingredients.