I think you are missing the point. At the moment, if the ball is going in and you handball it, you are sent off and it is a penalty. You gambled and lost. Under your scheme, you handball on the line when it goes in and you lose nothing. It is a goal anyway but there is also a chance the referee...
I have thought about that before but the main issue is there is no reason not to do it then. If the ball is going in anyway, you are not gambling anything by handling on the line. The referee might miss it and you get away with it. If they do see it, you don't lose anything as it would have been...