That is because of the possibility of cross body shots going through the upper arm then still having to enter the chest cavity. In a self defense situation you will not be making that shot or you will have to explain it.
Have you ever stood in a firing postion and looked at yourself in the mirror, or shot at a target depicting someone in Weaver firing stance? The adversary's gun and hands are blocking most everything below the eyes, and the forearms and upper arms are blocking the majority of the chest. You will not have a clear shot at CM unless you shoot below the breast bone/sternum.
That means a cross-body shot and/or a shot through forearm or upper arm. That is the reason for the FBI penetration standard.
A frangible round has a very poor chance of succeeding in this real-world scenario...which is why nobody uses or recommends them.
The real purpose of these rounds is to separate a fool from his money. At $3-4 per round, they are quite effective at that.
Here's the kicker:
If you shoot a revolver, you may be reasonably certain that the round will at least fire, although it's effectiveness is still in doubt.
If you shoot a semi-auto, consider this. Most thinking men recommend 200 trouble-free rounds of your chosen round before trusting it for carry. That number of Glaser/MagSafe would be $600-800.
If it were a 100% one shot-stop, vampire and zombie-killer, absolutely guaranteed to drop a raging bull elephant in it's tracks, it might be worth buying and carrying.
Considering the fact that it costs 3-4 (or more) times what premium SD rounds cost, and offers
no quantifiable advantage, and is arguably seriously deficient.... well, that makes the choice a simple one.