God tends to mess things up even more. When we get some principle right. When we achieve a certain level of spiritual growth. God does not just leave us in a state of joy. He takes us to the next challenge. Often that involves some suffering. Sometimes that suffering is quite intense. But that skews our results. So often you hear testimonies where people were living in sin and doing fairly well for a long time. Then they become Christian and a whole series of bad things happen to them. They are still happy they converted. They are experiencing a deeper joy and peace about life. They just have less superficial happiness. But if someone does a study they are going to show up as a Christian whose faith didn't produce measurable happiness.
In some way, if morality produces predictable happiness then it ceases to be morality. People living by the pain and pleasure principle will do it because it now meets that criteria. They might think they are more moral but that are not really. They are just smarter hedonists. True morality requires trusting some moral wisdom that not everyone is going to accept. An old song comes to mind:
That is at the root of it. We don't want to trust God and obey God. We want proof first. But asking for proof is the opposite of trust. Obeying only when you are convinced by other evidence is no obedience at all. So where is the proof? In the transformation of our own hearts. When we cooperate with God's grace and are made holy by God we become assured of God's existence and of our own salvation. There is the potential that others will see it to. Scientists are trained not to draw sweeping conclusions from a small data set. It is hard for modern man to break those rules. Still when people sense that someone they know is close to God they are drawn to them. Like God constructs a personal proof for every person. They fact that it is unlikely to be convincing to anybody else will make it hard. It will make it faith.