Actually, we only explode a die once. Even if we allowed it to continue, the 3d4 odds isn't 12.5%, it's half that at 6.25%
LOL it isn't as good as you think.
Even an (infinitely) exploding d4 does less average damage than a
non-exploding d6. And an exploding d6 does less average damage than a
non-exploding d8, and so on.
Mathmatically, an exploding die (with sides n) has an average of:
View attachment 381695
An exploding d4 averages 3.33
exploding d6 is 4.2
exploding d8 is 5.14
exploding d10 is 6.11
exploding d12 is 7.09
So, nice, sure, but not as great as people think. So, it is more psychological:
Dagger is a d4 weapon, but man, 25% of the time I get to roll an extra d4! Awesome, huh??
Much better than the 5% of the natural 20 IMO.
Finally, it also enforce the rule that an attack is binary. You succeed or you fail. If you need to roll an 8, it doesn't matter if you roll an 8, 18, or 20... They are all simply successful.
Now, when you roll the
damage die... That makes more sense IMO to be "critical" than the attack.