Because life is uncertain. We don't know what the future might hold.
If we knew the future, we'd calculate the reward we'll receive for each possible action, and choose the best one -- but alas, no one can tell the future. Therefore, we can't be sure what the reward of each possible action will be, and we can't be sure which action is best.
So, instead, for each action, we calculate the average of all possible rewards (weighted by their likelihood). Roughly speaking, this is our best guess at what the reward of the future might be, given the information available to us right now and the unavoidable uncertainty about the future. Then, we use that information to guide our decisions.