Because He wanted us to do it. Do you think abolition was wrong? Or civil/women's rights? Do you think He'd want us to sit on our thumbs and overlook genocide, starvation, etc? He had a singular job to do in a relatively short period of time: to bring the light into the world. It's up to us to ponder, reflect, and radiate that light.
"The only thing necessary for the triumph of evil is for good men to do nothing,"
I agree that Christians are not called to apathy or indifference toward suffering. Scripture is clear that love for neighbor matters and that faith expresses itself through love (Gal. 5:6). At the same time, the New Testament places a clear priority on the mission Christ actually gave His church.
Jesus’ earthly ministry was unique and unrepeatable—He came to reveal the Father and accomplish redemption (John 1:14; John 17:4). After His resurrection, He did not commission His followers to reform every social evil, but to proclaim reconciliation with God through Him (Matt. 28:18–20; 2 Cor. 5:18–20). The apostles lived under brutal injustice, poverty, and oppression, yet their primary calling remained the preaching of the gospel rather than organizing political or social movements.
That does not mean Christians should ignore suffering or refuse to help where they are able. Scripture encourages generosity, mercy, and care for those in need (Matt. 25:35–40; James 1:27). But it also cautions us against assuming that we can—or are called to—carry the weight of every global injustice. We are finite, and God has not entrusted every cause to every believer.
Our role, then, is not to “do nothing,” nor is it to shoulder every moral crisis of the world. It is to walk faithfully in the good works God has prepared for us (Eph. 2:10), while keeping the gospel central. Social good can flow from transformed hearts, but it is not a substitute for the message of reconciliation itself (Rom. 1:16).
In short, Christians are called to act in love where God places them, without losing sight of the primary mission Christ gave His church—to bear witness to Him.