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).
The soviet central committee tried this and failed miserably. the state run farms under produced the small private plots of land.
I've worked in a call center soliciting donations for a charity and found out that the charity only received 18% of the total donations the remainder was absorbed in administrative costs.
United Appeal is supposed to be a central agency for donations but again a majority of the funds goes to administration rather than to the agencies themselves. The charities would be worse off if run by the government than if allowed to work alone.
The I forgot or I didn't get around to it are weak excuses and a way of trying to say, "I didn't want to, so up yours" in politically correct language.If you really wanted to you would have gone out of your way to do so.
Look at the lotteries (1/3 goes to the winners, the remaining 2/3rds is supposed to go to charities but they actually get less than 1/6th due to administrative costs and other overhead.
The idea that the proceeds of the lotteries was to go to charities was only put in place to placate those opposed to gambling