Because they're voting for someone else to pay higher taxes to them. Most humans are incapable of delaying gratification for larger gains later. Voting for someone else to have to pay them money gets them something now. Voting for lower taxes takes time to benefit them. Plus, the lower taxes only benefit them if they're doing something productive.
This is why democracies always fail. You never heard of a democracy lasting as long as any nation state with a monarchy, or similar government.
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