Jesus never promised physical mercy in this life. In fact, he promised suffering for his followers. But he did promise mercy in the hereafter; it is spiritual mercy, which is much more important than this temporary physical realm.

This world has been under the control of Satan for almost all of human history, so we do see some physical mercy since anything good happens at all. Without God's intervention, nothing good would ever happen.

It's all about trusting God to know what he's doing, in spite of what Satan's doing. Keep your "treasures in heaven", not on earth.