There are a lot of reasons it can be reasonable to advocate higher taxes while not donating money to the government. Aside from what I already covered, it might be that a donation isn't reliable or predictable as ordinary tax income, so a donation in fact may not provide as much value per dollar as tax increases. You can't plan a budget around voluntary donations amounting to 0.1% of your total intake for the year, which weren't expected in the first place and may not show up next year. Consider the difference between winning $1200 on a scratcher ticket, versus $100/m increased income indefinitely. You may win $1200 again next year, but you might not, while a guaranteed $100/m is something you can count on and factor into your budget. Or the donor may perceive their peers to be free riding on the benefits their money provides (in the form of a more-stable society, et c.) if they simply donate the cash, so prefer taxation to donation. Or they may believe the reduction in power of the rich to influence society is part of the benefit of taxing the rich more, in the first place, in which case voluntarily reducing their own power is hardly a solution, and may even be counter-productive if that money could have funded advocacy for raising taxes on the rich, instead (or any of several similar reasons one, including a rich "one", may have to advocate for changes to the tax code that happen to increase one's own taxes, that aren't strictly related to increasing the government's budget).

At a level lower than the rich, it may be easier to follow: it can be consistent for me to believe that we should all pay more taxes to increase the budget for our school district, while not donating the difference myself, because I believe I need that money to cover deficiencies in their services until such time as a tax increase passes. Or, to take the harm angle again, consider the effect one person donating $500/m to schools has. $500/m is a lot of money for an individual (well, maybe not on HN, but for most folks it is). It hurts quite a bit to give that away. However, it does almost nothing for the district's budget, especially if they can't count on you continuing to donate it. Now, $500/household (I'm using simple figures to keep this clear, of course it never works like this exactly) in the whole district? By law, not just relying on people's whims? Now the schools can really do something with it (or waste it, but if you think they're just gonna waste it, you're also not gonna donate the money, so that's outside the scope of this).

As for 2: yeah, I was equivocating a bit :-) Still, in a world where marginal-utility sure appears to be a real thing as far as how people experience money, one person's one-millionth dollar really isn't the same thing as their first dollar, and it doesn't make much sense to treat dollars $1 and $1,000,000 the same just because some people don't have that millionth dollar in the first place.

Ultimately, one's view on progressive taxation will come down to what one believes fairness is, which is... harder to nail down than one might think, and reasonable people can disagree.