Why do Catholics pray to Mary and the saints? Isn't that idolatry?

Catholics do not worship Mary or the saints. Worship (latria) belongs to God alone (CCC 2096-2097). What we give the saints is honor (dulia), and to Mary the highest honor (hyperdulia), which is what Scripture itself does: "All generations will call me blessed" (Lk 1:48).

"Praying to" means "asking." In older English, "I pray thee" meant "I ask you." When a Catholic prays to St. Anthony, they are asking him to pray for them, the same thing you do when you ask a friend to pray for you. St. Paul asks for prayers constantly (Rom 15:30; Col 4:3), and no one calls that idolatry.

The saints are alive. God "is not God of the dead but of the living" (Mt 22:32). Those who have died in Christ are more alive than we are, not less. Hebrews 12:1 calls them "a great cloud of witnesses" surrounding us. Revelation shows the elders in heaven offering "the prayers of the holy ones" to God (Rev 5:8; 8:3-4). They are aware, they are praying, and they are on our side.

Why bother, if we can go straight to God? We can and do. But "the prayer of a righteous person is very powerful" (Jas 5:16), and no one is more righteous than those already in glory. Asking the saints is not going around God; it is going to God with the whole family (CCC 956-957, 2683).

Mary in particular. Jesus gave her to us from the Cross: "Behold, your mother" (Jn 19:27). At Cana her intercession moved Him to His first miracle (Jn 2:1-11). Every Hail Mary ends with the request: "pray for us sinners." That is the whole of Marian devotion: a mother praying for her children to her Son.