What a stupid question. That's like asking, why do Calvinists preach the gospel, after all, God already elected those who will be saved. Come on bro!
Actually, your answer is formed from ignorance, so I'll try to explain.
Calvinists proclaim the gospel to all men, as commanded by Christ. That's consistent.
Calvinists proclaim the gospel because the proclamation of the gospel is the means God uses to call His elect. That is consistent.
Calvinists proclaim the gospel promiscuously because we don't know who God's elect are. That is also consistent with our views.
Arminians attribute the ultimate choice as if it were in the hands of men. Men exercise their free will after God calls, draws, and provide prevenient grace to temporarily suspend the effects of original sin; therefore to ask God to do anything else in regard to a sinner is to ask for something inconsistent with your view.
I'm still waiting for someone to give me the precise language of their prayer in this regard.