Why do narcissists become so mentally unstable/ill, stop aging emotionally and mentally and whatnot, hence, they become so childish and act like toddlers?

I can't seem to wrap my head around this? They grow up to become some of the most immature and childish creatures, despite the fact that they could be 50 years old or older. Whenever you call them out on their bullshit, shut them down, or simply attack them in any way, they throw such a tantrum, when there are infinitely more solutions than just throwing a temper tantrum.

In general, during everyday life, they act so childish with their whining and complaining, and the things they do, like yelling out loudly unnecessarily or singing out loudly to the extent it annoys everyone, the things they do in everyday life is just pure childish behaviour. Why are they like this? Why do they stop aging emotionally and mentally?