Because it’s not true. Narcissistic personality disorder may included self loathing but it is not a requirement.

This pattern is shown by the presence of ≥ 5 of the following:

An exaggerated, unfounded sense of their own importance and talents (grandiosity) Preoccupation with fantasies of unlimited achievements, influence, power, intelligence, beauty, or perfect love Belief that they are special and unique and should associate only with people of the highest caliber A need to be unconditionally admired A sense of entitlement Exploitation of others to achieve their own goals A lack of empathy Envy of others and a belief that others envy them Arrogance and haughtiness

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Musk only needs 5. One could easily argue he has all of them.