Because you read it and substitute meaning for what you have been trained to think about it. This is the essence of Protestantism: reading the Bible not for what it says but for some imaginary false theology that is not there, the theology invented in 16th century by some charlatans in Germany. In this case, you read a clear reference in 1 Cor. 11 of the Christ's sacrifice being really present in the Eucharistic bread and the cup and you substitute for it your ideas of "being remembered, and celebrated".
The Body of Christ in palpable form was not in the bread and wine
Your position is without biblical merit. At the Last Supper Jesus said two things: (1) this meal is His body and blood and (2) the disciples are to "do it in commemoration" of Him.
Now, because I am an authentic Catholic Christian and not a Protestant, I take the Bible very seriously and I believe what it says fully. If Jesus said that the bread and the wine were His body and blood then I believe Him and not you who decides in his own head that because "Jesus was sitting there breaking, pouring and serving, talking, and his body was intact", then therefore "[H]e was not physically being sacrificed at the Last Supper". Jesus was present and also His sacrifice, His body and His blood were present in the Last Supper meal. This is not the only time the Holy Eucharist is described as a necessary for salvation actual meal being at the same time His actual blood and flesh, see John 6, second half of the chapter.
And likewise if Jesus tells his disciples to "do it" then I take that part seriously also. I do not imagine that anyone eating a matzot and drinking grape juice is "doing it" while not believing himself it is His body and His blood and His Holy Sacrifice. I don't deny that you are commemorating Christ, but the entire meaning of the Last Supper you pervert. I think in order to "do it" one has to be properly consecrated priest by a bishop, who comes in the unbroken line of succession from the Holy Apostles (2 Timothy 1:6, Titus 1:5) and believes the Gospel as written (Luke 22:19, john 6:51ff): the Eucharist is THE saving sacrifice of the Cross.
Jesus is not being re-sacrificed.
Of course not. The Holy Mass is not a re-sacrifice. The Last Supper was not a pre-sacrifice. Every Holy Mass is THE same sacrifice, "as often as you shall eat this bread, and drink the chalice". St. Paul said this clarification for a reason, because he knew there will be people like you imagining re-sacrifices. Jesus worked the miracle of multiplication of loaves and fish (the first half of John 6) before He explained the miracle of the Holy Eucharist (in the second half of John 6) for the same reason: because he knew there will be people like you denying His power.
Be ashamed of the Protestant faith-killing rot and flee it if you wish to be saved.
“Because you read it and substitute meaning for what you have been trained to think about it. “
That’s what I perceive you to be doing with it.
In 1 Cor. 11, you say there is a clear reference to Christ being actually present in the bread and wine. And there he is, actually serving the bread and the wine. He is bodily there, doing this.
And you say I am substituting meaning?
Discuss the issues all you want, but do not make it personal.