Why doesn't a defender use their hands in soccer?

It's not better than a for sure goal because the defender will be "stopping an obvious goal-scoring opportunity by handling the ball" which is the exact wordage for one of the possible reasons to give a red card. So, they would stop the goal, but a penalty kick would be awarded and that's as close to a sure goal as you're going to get, and on top of that they would be sent off and not be able to play the rest of the game (leaving his team shorthanded) as well as the next (meaning a starting defender's benched).