Because God is compassionate and interacts with his created beings in real and not just apparent ways. Since God is omniscient, to God these interactions happened in eternity past, but to us they are happening in the present. Therefore God makes no actual decisions in the present although to us it may seem that way. It also follows that our every evil action in the present, such as any that grieves the Holy Spirit, has real consequences even though those consequences were already determined in eternity past. But since we are finite, for all intents and purposes our actions have real consequences in the present and we will be held accountable for each and every one. To us our actions and their consequences have not yet been determined, but to God, who knows all our future actions and has already determined their consequences, they are already set in stone. The Calvinist, in my opinion, generally neglects the implications of God's attribute of omniscience in his interactions with creation, or, as it would appear to me, has God forming his determinative will with a limited or defective form of omniscience, although they would never admit to that.