I hear those complaints, but aside from a snarky "then spend your own money on then", they fail mention:
- anyone can spend money on things, if enough sub-billionaires spend money on something the same amount can be raised - there's no reason a billionaire needs to do it. On the other hand, large focused projects like this are arguably harder to crowdfund from a management perspective.
- The governments is richer than Musk. Taxation brings in more money, and is a much fairer way to contribute to these problems.
- the government is more empowered to deal with social issues. There are some things a private corp/citizen cannot do, at least without government approval anyway. The issues being tackled here are mainly engineering, that is "free" to solve. The items that they would "better" to fix are highly political, with a heavy side of "damned if you do, damned if you don't".
- throwing money at something can make it worse. A pipedream has no metrics that could prove this, so they can always be optimistic. When a "clothe the poor" drive kills local tailoring businesses, for example.
- Billionaires pay taxes, usually more than most (generally, though they have more leverage to bargain for lower taxes. unfair? sure, but that's global free trade gets you). What remains is income - why should they fix the problems government could, but won't?
- A major component of this is envy/belittling effort (Musk looks too happy in PR pics, so he's "playing around") or belittling the goal (spaceflight is useless) - i.e. grievance from the ignorant. There are a bajillion "useless" pastimes and occupations relative to some rando's pet social cause, yet billionaires are easy individual targets when everyone is equally to blame - at least Musk probably works hard.