Why would hardness of hearts prompt this?
Because flesh (used in Scripture to represent sin, unbelief) cannot understand spirit and life, as Jesus here, and Paul elsewhere indicates.
Then they said to him, “What must we do, to be doing the works of God?” Jesus answered them, “This is the work of God, that you
believe in him whom he has sent.” (28-29)
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." (35)
Jesus = bread; coming = eating; believing = drinking; coming and believing in Jesus = eating and drinking from him.
But I said to you that you have seen me and
yet do not believe. (v 36)
For this is the will of my Father, that everyone who looks on the Son and
believes in him should have
eternal life, and I will raise him up on the last day.” (40)
So the Jews grumbled about him, because he said, “I am the bread that came down from heaven.” They said, “Is not this Jesus, the son of Joseph, whose father and mother we know? How does he now say, ‘I have come down from heaven’?” (41-42)
First misunderstanding by the crowd. Jesus says he is come down from heaven. Their minds immediately turn to his carnal, earthly birth. Jesus speaks of the Incarnation.
“Truly, truly, I say to you, whoever *believes *has eternal life.
I am the bread of life.” (47-48)
"This is the bread that comes down from heaven, so that one may eat of it and not die. I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever. And the bread that I will give for the life of the world is my flesh.” (50-51)
Jesus is the bread. If anyone eats - comes to him - they will not die. Have eternal life.
The Jews then disputed among themselves, saying, “How can this man give us his flesh to eat?” (52)
Second misunderstanding. Again, their minds cannot perceive what Jesus is meaning with his metaphor because they are trapped in the flesh and cannot understand/see the kingdom of God.
So Jesus said to them, “Truly, truly, I say to you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever feeds on my flesh and drinks my blood has eternal life, and I will raise him up on the last day." (53-54)
Same thing that Jesus says in verse 35; only now by way of figurative language. Whoever comes and believes in the bread from heaven will have eternal life. If there is no coming and believing…there’s no life in you.
It is the Spirit who gives life; the flesh is no help at all. The words that I have spoken to you are spirit and life. But there are some of you who
do not believe.” (63-64)
The entire section is book ended by faith that sets the entire beginning and ending context. Again, this doesn’t disprove the real presence. It just means we have to let the text give us the understanding of when Jesus is using metaphor. A dogma in search of a proof text is a pretext for a bad context.