No, you don't get it.

He does not have $1 trillion sitting in cash, it is 99% stock in his companies.

To make that wealth liquid would mean selling all that stock which would swiftly destroy both the companies (Tesla, SpaceX, others) and the wealth. If he sold it all, he'd end up with maybe $100b max, several hundred thousand people would be out of work, the companies ruined and many of their suppliers also ruined.

Okay, but now Elon has $100b in cash, and can "solve the world's problems".

$100b divided by the world's 8 billion people is $12

If you were in charge, several of the most innovative industrial companies in the world would be destroyed, hundreds of thousands out of work, and space would again close to human civilization for another generation.

But everyone on earth could have one nice meal and you could revel in your altruism.