You're wrong most walk when they're bowled or sky the ball. The rules state all decisions even being bowled has to be appealed and the umpire signal out.You could probably count the number of current international batters who consistently walk on one hand.
If you consider that cheating (and to be clear, there's nothing in the rules to say anybody has to walk if they think they've hit it) you might as well not bother with the whole game.
Although technically an appeal is required for the umpire to make a decision, in practice it is often obvious to all that a batter is out, and the batter may walk off the field without waiting for the decision of the umpire. This is often the case when a batter is out bowled or to an obvious catch. However, the batter is always entitled to stand their ground and wait for a decision from the umpire. In cases where they consider they might not be out, such as a catch taken low near the grass or where it is not clear whether the ball hit the bat, batsmen will not take the walking option. It is then up to the fielding team to appeal for a decision. Sometimes a batter will walk even when it is not clear to others that they are out, if in their own mind they are certain they were out; this is considered to be the epitome of sportsmanshi