Because I believe it is simpler, more efficient, and more fair if a centralised body deals with the distribution of funds. If there was some organisation that did this instead of the government, I would be equally happy to use it, but the benefit if including it in taxes is that everyone pays and there's no forgetting or 'getting round to it sometime' involved (never underestimate the power of apathy).
In an ideal world, that's true. But less money reaches the charity after all of the bureacracy, and the money that you "donate" could go to help someone 2000 miles away instead of improving your own community.
If my tax rate was 10% instead of 30%, I'd definitely give more to local charities. Yes, there is a risk that the homeless kittens get more support than the homeless men, but I think the reason people donate to the kittens now is because their taxa dollars are already going to support the homeless men (at least that's my personal reasoning).
The country either needs to rewrite the constitution and make it a complete socialist government (i.e. higher taxes with more social programs) or go back to their roots and treat the federal government like the EU plus a military.