It's because catholics believe in the immaculate conception of Mary. They believe that God chose a sinless vessel to give birth to a sinless Jesus. What they ignore are the scriptures that state "For all have sinned and fallen short of the glory of God" (Romans 3:23). Only Jesus was sinless (the second Adam). No other. And God did not need a sinless vessel to give birth to a sinless Jesus, since Jesus was conceived by the Holy Spirit.
Based on this doctrine of theirs, to say Mary was sinless is to suggest she is equal to Jesus (even if catholics can't see this).
Also, the Hail Mary prayers are a request for intercession: "Hail Mary, full of grace, the Lord is with thee; blessed art thou amongst women, and blessed is the fruit of thy womb, Jesus. Holy Mary, Mother of God, pray for us sinners, now and at the hour of death. Amen"
The extra books in the catholic bible are what they use to support many of their teachings. But they also refer to the scripture that says to pray for each other, and because the "saints" are in Heaven, they don't believe praying to them is wrong, and they cite scriptures like the one that speaks of God not being a God of the dead, but a God of the living.
Copy/Paste Catholic Encyclopedia...
"The Hail Mary (sometimes called the "Angelical salutation", sometimes, from the first words in its Latin form, the "Ave Maria") is the most familiar of all the prayers used by the Universal Church in honour of our Blessed Lady.
It is commonly described as consisting of three parts. The first, "Hail (Mary) full of grace, the Lord is with thee, blessed art thou amongst women", embodies the words used by the Angel Gabriel in saluting the Blessed Virgin (Luke, I, 28). The second, "and blessed is the fruit of thy womb (Jesus)", is borrowed from the Divinely inspired greeting of St. Elizabeth (Luke 1:42), which attaches itself the more naturally to the first part, because the words "benedicta tu in mulieribus" (I, 28) or "inter mulieres" (I, 42) are common to both salutations. Finally, the petition "Holy Mary, Mother of God, pray for us sinners now and at the hour of our death. Amen." is stated by the official "Catechism of the Council of Trent" to have been framed by the Church itself. "Most rightly", says the Catechism, "has the Holy Church of God added to this thanksgiving, petition also and the invocation of the most holy Mother of God, thereby implying that we should piously and suppliantly have recourse to her in order that by her intercession she may reconcile God with us sinners and obtain for us the blessing we need both for this present life and for the life which has no end."