Because his kingdom is not of this world. (I thought some would enjoy that 'witty' answer.)
On a serious note, your comment implies that my action is simply voting for 'anyone' who represents my values. However, that is an entirely mis-conceived belief. I'm still voting for a presidential candidate; an individual who is certified (though maybe not in every state) to receive votes in this election. This individual has also stated that he would take the role if [somehow] voted in.
The comparison you made, while I understand your train of thought, is entirely flawed. If Ron Paul wasn't certified or said "I would not assume the role if voted in", I obviously would not be voting for him. However, he is a candidate (maybe not in your state); thus I will vote for the candidate who best represents me.