Because there are multiple sins. That does not multiply Calvary. The Mass applies the one historical sacrifice to the faithful. When the Eucharist is consecrated, the language is both in the past tense and in the present tense, and it is straight from the scripture:
Priest: ...On the night he was betrayed, he took the bread and gave youthanks and praise. He broke the bread gave it to his disciples, and said, “Take this all of you and eat it; this is my body which will be given up for you.” ...When supper was ended, he took the cup; again he gave you thanksand praise; gave the cup to his disciples and said, “Take this, all of you, anddrink from it; this is the cup of my blood, the blood of the new and everlastingcovenant. Do this in memory of me.Priest: Let us proclaim the mystery of faith.
Everyone: Christ has died, Christ is risen, Christ will come again.
Priest: Through Him, with Him, in Him, in the unity of the Holy Spirit, allglory and honor is yours, almighty Father, forever and ever.
Everyone: Amen.
Priest: Let us pray for the coming of the kingdom as Jesus taught us.
Everyone: Our Father...
Priest: Deliver us Lord from every evil, and grant us peace in our day. In your mercy keep us free from sin and protect us from all anxiety as we wait in joyful hope for the coming of our Savior Jesus Christ.
Everyone: For the kingdom, the power, and the glory are yours, now and forever.
Priest: The peace of the Lord be with you always.Everyone: And also with you.
Priest: Let us offer each other a sign of peace.
Priest: This is the lamb of God who takes away the sins of the world.Happy are those who are called to his supper.
Everyone: Lord I am not worthy to receive you but only say the word andI shall be healed.
Further, the Mass is a celebration of sacrifice and through it the victory of Christ. It does not itself absolve any sin, at least not any serious sin, for that a separate sacrament exists, Confession.
I may be confused by terminology. You write:
“Further, the Mass is a celebration of sacrifice and through it the victory of Christ. It does not itself absolve any sin, at least not any serious sin, for that a separate sacrament exists, Confession.”
The Council of Trent said:
“CHAPTER II. That the Sacrifice of the Mass is propitiatory both for the living and the dead.
And forasmuch as, in this divine sacrifice which is celebrated in the mass, that same Christ is contained and immolated in an unbloody manner, who once offered Himself in a bloody manner on the altar of the cross; the holy Synod teaches, that this sacrifice is truly propritiatory and that by means thereof this is effected, that we obtain mercy, and find grace in seasonable aid, if we draw nigh unto God, contrite and penitent, with a sincere heart and upright faith, with fear and reverence. For the Lord, appeased by the oblation thereof, and granting the [Page 155] grace and gift of penitence, forgives even heinous crimes and sins. For the victim is one and the same, the same now offering by the ministry of priests, who then offered Himself on the cross, the manner alone of offering being different. The fruits indeed of which oblation, of that bloody one to wit, are received most plentifully through this unbloody one; so far is this (latter) from derogating in any way from that (former oblation). Wherefore, not only for the sins, punishments, satisfactions, and other necessities of the faithful who are living, but also for those who are departed in Christ, and who are not as yet fully purified, is it rightly offered, agreebly to a tradition of the apostles.”
http://history.hanover.edu/texts/trent/ct22.html