Well the reason is that there are degrees of success and failure. The Calvinist, well at least anyone who actually knows some theology, will say that even man's best actions are flawed.
Take for example John Calvin's prayer: "We are poor sinners conceived and born in iniquity. Prone to do evil, incapable of any good. And in our depravity we transgress Gods holy commandments without end or ceasing."
You would be quite wrong in thinking that Calvin would feel a need to lay down and give up. He cannot keep the law for one second! That's the point: he has to put his faith in Christ he has nothing in his credit to show to God. Love is the only correct response to such a free gift from God. Thus the Christian life will be one of loving God and brokenhearted humility that God chose me while I had no good actions no not one to my credit. But, the point is that we have Christ's righteousness in our account, no sin credited to our name.
Romans 3:10-12:We have nothing to give to God and nothing to commend ourselves to him. Because of his gracious gift to us while we had absolutely nothing to merit it our hearts turn in love to him. The question you ask though is why we shouldn't give up since all our actions are flawed. Well God calls us to the task of following his commands and if he calls us to an impossible task because we love him we should do it. The Calvinists sees degrees of success and failure. Sincere heartfelt prayers are truly pleasing to God despite their falseness. As Jesus puts it: "if you love me you will do what I command." Doing what he commands of course has no effect on our salvation, because we have Christ's righteousness in us and no sin to our account. God in the bible is often written about as looking upon his servants with favor. This does not negate in the least the fact of the fact all these men's actions are fallen. Calvinism is often misunderstood because people will say that if we are saved by faith alone why not go on sinning? We reply, no we shall not go on sinning that Grace may abound. (Paul had to deal with this one, in Romans 6 maybe that is a good place to look for your answer.) Basically a life where one rests in the comfort of "I believe in God and am thus saved" and has no sign of works in their lives is suspect. See my entry on June 2 because that is what that entry is about. Basically for the Calvinist our great depravity is not to be something to discourage our effort, but it is always used to magnify the intensity of God's glory.
As it is written, "None is righteous, no, not one;
No one understands;
No one seeks for God;
All have turned aside, together they have become worthless.