Because combat is inherently exciting in a way that most things are not. Combat naturally engages the mind, it changes the world in a visceral and immediate and often flashy way, and it ties in neatly with the narrative concepts of conflict and struggle. Video games are uniquely well-suited for combat. It isn't the only thing they can do, certainly, but I don't think it's fair to suggest that having combat in a video game is a failure of creativity. Combat just takes up a really large portion of the creative space there. Similarly, dramatic films have a strong tendency to focus on humans despite the fact that there are narratives that could be constructed around non-human entities.