Just having hundreds of millions of dollars wouldn't be particularly exciting once the novelty wore off. The exciting part is earning hundreds of millions of dollars via the immense success that it implies/requires.
It's very strange how people now seem to feel like they are entitled to see the tax returns of public or well-known figures. Tax returns are nobody else's business! Would you demand to see their medical records as well?
The implicit subtext (especially with highly successful people like Musk) is that we do this to "make sure they pay their fair share". Here's the reality: the top 1% of US tax filers pay nearly half of all US income taxes[1]. The bottom 80% of filers (by income) pay only 15% of all taxes. This myth that the rich don't pay taxes is completely, utterly false. It is a myth promulgated for political purposes.