Because that would be a tax on empathy. We shouldn't be taxing socially positive traits like empathy, but instead taxing negative traits like greed. In lieu of a moral balance for weighing souls, I'll accept a tax on accumulated wealth instead.

I agree with the sibling comment, but there's also a practical reason: I don't want to have to track down everyone that needs assistance and make a personal decision about who needs what. It's much more efficient to have a tax collector and bureaucracy.

For the voluntary vs involuntary part of taxes and charity, that's a moral/ethical philosophical argument that people have been debating for thousands of years. I don't expect to convince anyone here, but I think my view is pretty clear.