Part of it is because the cause of my choice is not where the money should go. I want a functional safety net, good schools, strong infrastructure, and a million other things to ensure a healthy society. I'm not sufficiently educated to donate money in a way that ensures all that stuff gets funded. I accept that some inefficiencies exist but also the government is funding things I need but have no idea about.

Also, I can make only so much impact on my own. Advocating for taxes gets many people to give.