To be fair I didnt actually know you could do that, although donations to government would be right at the bottom of my list. I suspect most don't know it's possible.
The bolded bit is spot on in truth. Why would anyone give extra in taxes when they could give directly to a cause where their funds actually make a difference?
Which also leads to another point. Reducing people's liquidity inevitably leads to a reduction in their ability to give to charity.
Talk about a specious argument... Tax only works when lots of people pay it together and do so predictably.
If charities, scholarship programs and various other causes for good in society can run successfully and perpetually whilst raising millions and helping millions on the donations and goodwill of the general public; without the threat of prison time for non-payment... Why wouldn't government be able to harness the apparant fact that millions of people are happy to pay billions extra in tax?
I can answer that question: firstly because government are inefficient and bureaucratic when it comes to spending money so very quickly people would decide they were getting very little bang (in the form of helping people) for their buck. Secondly these people who're eager to spend thousands extra in tax don't exist (in any meaningful numbers), from experience it's a theoretical argument generally made by people who're arguing that people other than themselves are happy to pay more.
People who're happy to spend their money in the pursuit of helping others already doing so in the form of charity. Legislating for them to have to redirect their goodwill, forcing them to spend their money instead on Velodromes, the Theatres Trust or the salary of people working at the House of Lords Appointment Commission instead of causes that are generally close to their hearts is outrageous.