I’ve always wondered this, too.
The creation stories of Genesis are usually not taken literally. I have never met anyone who does. Various arguments are that it is ridiculous that the human race could come from a genetic origin of just two people, it flies in the face of everything we now know about evolution, there would have been incest committed, etc.
So, if I claim that the Genesis stories are “faith stories” which attempt to make a wider point about the role of God in creation, the relationship between humanity and God, and the inherent sinfulness of humanity, I would not be kicked out of the church for believing that. Indeed, I would be regarded as a normal, modern Catholic.
Now, imagine if I suggested that John 6 (the “flesh and blood” statements) should not be taken literally because it flies in the face of everything we know about chemistry and physics, it’s gratuitous and unnecessary, and it goes beyond cannibalism to take the substance of God into your digestive system. I’d be dismissed as a heretic, someone unwilling to bend to the church’s interpretation. The literal interpretation of John 6 is absolutely, staunchly required of all the faithful.
It seems that the church is happy to accept various readings of one passage, while betting the house on another passage.
I don’t think we can pick and choose as it suits us. At the very least, we should not force an interpretation on anyone. As was noted a few posts above, many people take the Genesis creation stories as literal fact. Let them believe it. Let others believe that God’s flesh is eaten and his blood drunk in a bizarre ritual, performed every minute of every day.