Because through voting, you give legitimacy to the state to apply its law to you, regardless of who you vote for or who wins. Even if you cast down a blank vote.

Let me elaborate:

You say taxation is theft. Many people reply with “Taxation is the fee you pay to live in a civilized society kept together through the social contract.”

But you didn’t sign any contract. Oh wait, did you vote this last election? Yep, that’s the contract and you signed voluntarily. Now pay up 50% of your income to stay out of jail you well behaving citizen.