Because:
1.) We are told to by Jesus. Period. That in and of itself should be enough for anyone.
2.) Because we do not know who are and are not the chosen of God. Just because there is a chosen people doe not mean we can say who is and who is not one of the chosen. Therefore, we go out and evangelize. You see, your premise is that if the power of Evangelism is not [as per everything else] in your hands, then it can't be effective. I put the power of Evangelism in God's hands, where it belongs.
So, you continue to ask, "Why even bother talking to people, hmmmmmm? If there is no free will, then all is pointless. Not so. Because although I do not have a free will, I have a God with a Sovereign will. Read Isaiah 6:8-13 . Isaiah is told to preach, but that God will harden the people hearts until they are all wiped out and there is nothing left. He is telling Isaiah what will happen, and yet telling him to preach anyways.
So, after re-re-re-re-re using that same question here is your answer you claim we could not give.