Why do Catholics repeat prayers when Jesus said not to use vain repetition?

Jesus did not condemn every repeated prayer. He condemned empty babbling that treats prayer as a technique for manipulating God or impressing other people.

What Jesus actually condemned. In the Sermon on the Mount, Jesus warns against praying for public praise and against heaping up empty phrases as the Gentiles do, thinking they will be heard because of their many words (Mt 6:5-8). His concern is vanity, superstition, and a false idea of God. The Father already knows what we need. Prayer must come from faith and an attentive heart, rather than from the belief that enough syllables will force a divine response.

Repetition can be biblical. Immediately after this warning, Jesus gives His disciples the Our Father, a set prayer meant to be prayed repeatedly (Mt 6:9-13). In Gethsemane, Jesus prayed a second and third time, saying the same words (Mt 26:42-44). Psalm 136 repeats the refrain, “for his steadfast love endures for ever,” throughout the psalm. In heaven, the living creatures continually proclaim, “Holy, holy, holy,” before God's throne (Rev 4:8). These examples would make little sense if all repetition were forbidden.

Why Catholics use repeated prayers. The Our Father, Hail Mary, Jesus Prayer, litanies, and Rosary give the mind and heart words rooted in Christian faith. Vocal prayer is an essential form of Christian prayer because human beings pray with both body and soul (CCC 2700-2704). In the Rosary, the repeated Hail Mary creates a steady rhythm while the person meditates on the mysteries of Christ's life. The Rosary is centered on Jesus, from His Incarnation and ministry to His Passion, Resurrection, and glory.

When repetition becomes vain. Any prayer can become inattentive or mechanical. A person might recite a Rosary while deliberately refusing any effort to pray, or say many prayers merely to look religious. Yet distraction by itself does not make prayer worthless. When the mind wanders, gently return to the words or mystery. Persevering through distraction can itself be an act of love.

Pray repeated prayers slowly enough to mean them. Before beginning, place yourself in God's presence and name the intention you are bringing to Him. If attention wanders, return without anxiety. God seeks a sincere heart, not flawless concentration or an impressive number of words.