Why do Catholics confess their sins to a priest instead of directly to God?

Catholics do confess directly to God. Every Mass begins with it, and the Church urges daily contrition. But for mortal sins after Baptism, Christ established a sacrament, and the priest is the way He chose to deliver His forgiveness.

Where it comes from. On Easter evening the risen Jesus breathed on the apostles and said: "Receive the Holy Spirit. Whose sins you forgive are forgiven them, and whose sins you retain are retained" (Jn 20:22-23). That is a commission to judge and forgive, which only makes sense if the sins are actually told to them. Jesus had already promised Peter and the apostles the power to bind and loose (Mt 16:19; 18:18), and St. James tells the early Church to "confess your sins to one another" and to call the presbyters (Jas 5:14-16).

Why God would do it this way. The Catechism explains that sin wounds both our relationship with God and our communion with the Church (CCC 1440). Reconciliation therefore has a visible, ecclesial shape. The priest acts in persona Christi: it is Christ who absolves, using a human voice so you can actually hear "your sins are forgiven" and know it happened (CCC 1441-1442, 1465). Confession also gives you counsel, a penance, and the humility of saying it out loud, which is itself healing.

What the early Church did. The Didache (c. 70-100) says to confess sins in the assembly. St. Ignatius of Antioch, St. Cyprian, and Origen all describe confession to the bishop or priest. The practice of private, repeatable confession spread from the Irish monasteries and was universal by the Middle Ages, but the sacrament was there from the beginning.

In practice. Venial sins can be forgiven many ways: prayer, the Eucharist, acts of charity. Mortal sins require the sacrament before receiving Communion (CCC 1457; Canon 916). If it has been years, go anyway. The priest will help you, and nothing you say will shock him.