[lotsofpulp]

Because a government can be held accountable to its voters, and laws about transparency and equal access whereas private charities can't. Given the option, people will chose to transfer their wealth to those in their own tribe, and not to those outside of it.

[nickff]

Government accountability is a tricky thing, as you have a limited number of choices, each of which has a basket of different positions on a very large number of issues. Since you can only make one (of a few) choice(s), voters each have a very limited ability to hold the government accountable on a few issues.

Charities are held to account by a very different constituency (the donors), who are in a much more powerful position.

I'm also not sure your assertion that "people will chose to transfer their wealth to those in their own tribe, and not to those outside of it" holds much water. I think donors often give to causes which affect people very different from themselves, and anyway, your state or nation is really just a different sort of tribe. Given some convincing evidence, I might believe that state-welfare programs are more equitable than private charities, but I have not seen such evidence.