Why does transubstantiation refer only to communion? Why does the doctrine of transubstantiation consider Jesus’ words regarding the bread and wine of communion to be literal while considering these other similes used by Jesus to be metaphorical?
Alleged Literal

Matthew 26:26-27 ESV  Now as they were eating, Jesus took bread, and after blessing it broke it and gave it to the disciples, and said, “Take, eat; this is my body.” And he took a cup, and when he had given thanks he gave it to them, saying, “Drink of it, all of you, for this is my blood of the covenant, which is poured out for many for the forgiveness of sins.

Alleged Metaphorical

John 15:5 ESV I am the vine; you are the branches.
John 1:29 ESV The next day he saw Jesus coming toward him, and said, “Behold, the Lamb of God, who takes away the sin of the world!
John 6:35 ESV Jesus said to them, “I am the bread of life; whoever comes to me shall not hunger, and whoever believes in me shall never thirst.
John 8:12 ESV Again Jesus spoke to them, saying, “I am the light of the world. Whoever follows me will not walk in darkness, but will have the light of life.”
John 10:7 ESV So Jesus again said to them, “Truly, truly, I say to you, I am the door of the sheep.
Matthew 12:50 ESV For whoever does the will of my Father in heaven is my brother and sister and mother.
Acts 4:11 ESV This Jesus is the stone that was rejected by you, the builders, which has become the cornerstone.

This question is directed towards those that accept the alleged literals/metaphoricals listed above. That would be Catholics, some Lutherans, etc.
The beginnings of a response would consider the difference between a simile/parable (&quot;The kingdom of heaven is like...&quot;), a metaphor (&quot;I am the vine&quot;), and a prescription or statement of fact (&quot;Blessed are the pure of heart&quot;). Everyone would agree that Jesus employs a wealth of literary and rhetorical devices to communicate his beautiful message, and any attempt to argue otherwise would be hard-pressed to explain what the &quot;Hand of God&quot; could refer to in the Psalms. Origen has excellent things to say about the different levels of meaning of Scripture.
As for this particular instance: I will respond despite the fact that it is obviously posed as a &quot;loaded&quot; question.
Jesus uses &quot;ego eimi&quot; (I am) many times throughout the New Testament, either alone to refer to His divinity (&quot;Before Abraham was, I am&quot;), or to indicate his present (&quot;Do not fear, it is I&quot;), or to predicate something of himself. In the Gospel of John, especially, he does this numerous times: &quot;I am the vine,&quot; &quot;I am the bread of life,&quot; &quot;I am the good shepherd,&quot; &quot;I am the gate,&quot; etc.
I think it is very difficult to understand the four extant accounts of the Last Supper &quot;Consecration&quot; (as Catholics would refer to it) in Matthew, Mark, Luke, and St. Paul without hearkening back to a chapter that you mention in passing in your list of &quot;Alleged Metaphors,&quot; and that chapter is John 6. It merits a long, slow read and reveals several characteristics which should make us pause before receiving it as merely metaphorical. Why?

Christ's insistence. He states &quot;I am the bread of life&quot; numerous times in several different ways in a way that makes the &quot;Good Shepherd&quot; discourse look like a short excursus.
His concreteness. Though &quot;bread of life&quot; easily argues a merely metaphorical reading, it is difficult to understand why Jesus not only repeats but also intensifies this statement to something that two millenia of Christian culture has not been able to make tame: &quot;Unless you eat my flesh and drink my blood...&quot; And then again, the same repetitive insistence.
The scandal. Perhaps most significant of all, the above reading of the statement as scandalously literal is all the more convincing given the reaction of the Jews themselves: they are scandalized and they scatter. Did they misunderstand Jesus? If so, could not Jesus have added a brief little note at the end, &quot;You're not taking this the right way&quot;? When he addresses the apostles, he does not say, &quot;You understand what I mean, right?&quot;; no, he says, &quot;Will you leave me too?&quot; Something big is at stake.

And then, the Last Supper. &quot;This is my body; this is my blood.&quot; If this were an isolated image, again it would make more sense to read it metaphorically. Combined with the words of His ministry, though, it seems logical to ask: Is this the same body and blood that we are to eat and drink?
Perhaps we could get this far and still find it appealing to refer to the bread and wine as mere symbols, not substantially changed into the body and blood of Christ, as Catholic Transubstantiation says. Why so literal? The answer here hearkens back to a continual point of tension between the Catholic Church and certain other denominations over the proper place of Sacred Tradition. What did the early Christians do? What was their understanding of what they were doing? One first clue might be the continual suspicion under which the early Christians fell in the first few centuries of their existence under the Roman empire. They were continually accused of...anthropophagy: &quot;eating human flesh.&quot; A curious accusation against a group of people who symbolically broke bread together.
The testimony of the Fathers is not unequivocally unanimous, but it is strongly in favor of the real presence. Some citations from the first and second centuries:

For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. (Source: Justin Martyr First Apology, 66)
Take care, then, to use one Eucharist, so that whatever you do, you do according to God: for there is one Flesh of our Lord Jesus Christ, and one cup in the union of His Blood; one altar, as there is one bishop with the presbytery and my fellow servants, the deacons. (Source: Ignatius of Antioch, Letter to Philadelphians, 3:2-4:1)
[Christ] has declared the cup, a part of creation, to be his own Blood, from which he causes our blood to flow; and the bread, a part of creation, he has established as his own Body, from which he gives increase to our bodies.&quot; (Source: St. Irenaeus of Lyons, Against Heresies, 180 A.D)

My favorite testimony of the Fathers comes from St. John Chrysostom, who uses a graphic image to describe how literally he understands the Transubstantiation:

When the word says, ‘This is my Body,’ be convinced of it and believe it, and look at it with the eyes of the mind. For Christ did not give us something tangible, but even in His tangible things all is intellectual.... Since the soul is intertwined with the body, He hands over to you in tangible things, that which is perceived intellectually. How many now say, ‘I wish I could see His shape [Gr. ton tupon], His appearance, His garments, His scandals.’ Only look! You see Him! You touch Him. You eat Him. He had given to those who desire Him, not only to see Him and fix their teeth in His flesh, and to embrace Him and satisfy all their love. (Source: St. John Chrysostom Homily on Matthew 82.4)