I want tax to be planned & used effectively.
If people make random donations, its unpredictable and can't be relied upon, it can be assumed as a cashflow for several years that can fund meaningful investment in services or capital projects the way that tax revenue can.
Honestly we all just need to pay a little bit more. Everyone. It is much, much easier to raise big sums by sticking a penny on the basic rate or on vat, than it is to try and put in a new wealth tax or higher rate band. For most lower income people the resulting improvement in public services etc is worth it.