You lowering your tax rate and giving that money to charity isn't magicking more money into the world, it is just a different allocation.
The government's tax income is allocated by the masses (in theory anyway). It is fair and dispassionate.
Philanthropy / charity is picking winners and losers based on your personal whims, and for many it is about gaining social capital.
In order to improve global health or address other issues that impact countries beyond where you live, the government (even an idealized version without waste, corruption, or political games) might not be the most effective way to accomplish this.
What I find kind of interesting is that Bill Gates and Warren Buffet argue that they should be taxed more, but they don't do anything to further that goal aside from media soundbites and headlines. They could fund an incredible war chest for a lobbying apparatus who's sole purpose would be to create a more fair tax system. But no such thing happens.
It seems fortunate that some charitable funds isn't subject to that risk. There are other risks, but they seem lower.
In an imperfect world, government funding alone seems insufficiency diversified. That's too much power in one place.
This is only ever true if you assume that government tax spending is 100% efficient, with nary a fraction of a cent being wasted. I don't think that's a safe assumption.
Let's say you just won the startup lottery and you've got a significant amount of now tradable stock. Some of which was early exercised and the cost basis is effectively zero. Some of which was RSUs or non-qualified options and you owe ordinary income. And that you're way over into the top tax brackets.
If your zero cost basis stock is Qualified Small Business Stock (QSBS), there's a very nice discount on federal capital gains, so you might not need to do the rest of the stuff.
Otherwise, if you donate your apprechiated zero basis stock, you get to save federal capital gains of 20% + 3.8% net investment income. Plus it offsets against your ordinary income that's 37%. So that's a 60.8% discount on being charitable for the feds. If you live in California, capital gains are regular income, so you're saving 13.3% because the capital gains go away and offsetting 13.3% on your ordinary income, so your total discount is 87.4%. In other words, your difference in cash after taxes for selling $1M of zero basis stock or donating $1M of zero basis stock is $126k.
When the government is telling you it only costs $126k to give a charity $1M, it's pretty compelling. The math used to be different, when you'd get credit for state taxes on the federal return, but that was many years ago now.