Not all contraceptives prevent; some cause abortions. The bigger picture for the Church is that we Catholics are to trust God, and to believe children are His gifts to us...we are not in control, He is.
Besides, once we decide to use contraceptives and they fail--then what? Do you say, well I guess abortion is the only solution to my problem now? God's way eliminates all those difficult, heart-rending decisions when we choose to live as the Church teaches. We know that babies often come from the sexual act, and as married couples we are taught to be lovingly accepting of children for our family's continuation.