That’s because it was so obviously meant metaphorically. No one there expected to eat flesh or drink blood. And they didn’t, either. There is not need for him to add,“And I don’t mean this literally.”
Now, just run this line of reasoning through all of John 6 and let’s see how it plays out. This chapter of John can be divided into three parts - (1) feeding the 5,000, (2) calming the storm and, (3) Eucharistic Discourse … or… the ‘Misunderstood Metaphor’ as you may want to rename it…
Now,it could be that there really weren’t 5,000 people there (much less 5,000 men). This number is really a metaphor for how we hungar for Christ. There was just a few more then the 12 Apostles who had probably already eaten, And, Christ sees this handfull of hungry people and this kid shows up with some food … and they were careful to share and portion out the food that was presented … well, everyone got something to eat. Now, the 12 baskets of fragments was obviously a metaphor for the 12 Tribes of Israel - and they were a metaphor for the 12 Apostles. That wasn’t too hard, eh?
Now the storm is going to a bit harder of a metaphor … but, in keeping with your approach - here goes: Christ had really gotten into the boat without the Apostles knowing it (maybe He hid under a sail or something) Well, this mild rain shower comes in on them … and some waves and just as the storm was going to pass - Christ appears in front of them (a metaphor for us not knowing the day or the hour when we will meet Christ…) and this scares the Apostles who think they have seen a ghost! Peter is obviously so upset, he falls overboard (no one can walk on water - another metaphor, I’m sure) and Christ lends a hand and pulls the old man back in the boat. Now, this is an obvious metaphor for how we all will surely sink without Christ’s Hand to bring us to safety!
How am I doing so far?
And, since you have handled the part about, “My Flesh is REAL food and My Blood is real drink” as either a misquote or a metaphor for something else… we have it all explained away!
But, as I withdraw my tongue from my cheek… I am left wondering - never in any of the Gospels did people become offended by a metaphor used by Christ. While some may have been offended with Him calling Himself a: door, vine, sheepgate, sower of seeds, good shepherd, etc - the departure of these offended souls is just not recorded. So, here we have this issue - why would they be offended if called Himself a loaf of bread?
The answer is - no one thought he was calling himself a load of bread - they thought He was crazy! They understood what He had been saying. How do I know this? Simple. These 1st Century Jews (and their followers in the 16th Century…and, then their followers…) said, “How can this man give us his flesh to eat?”
Notice, they did not wait around for the answer. This rejection type response is a little different from the 16th Century crew who simply rejected the Church founded by Christ on Peter, threw out the guidance given by the Holy Spirit for the past 1600 years, abridged the Bible that had been approved since the 5th Century and decided to follow men who claimed to be following Christ rather then Christ through the Church He founded and lead by the Holy Spirit. You see, there’s the rub. One either conforms one’s behavior to match one’s belief - or we just change our belief to conform to our behavior.
The statements Christ uttered in: John 6, in the Last Supper Narrative and as reported by Paul in a private revelation as recorded in 1st Corinthians either are the inspired Word of God to be taken literally or they are a big joke! From the founding of Christ’s Church on that First Pentecost Sunday - the Catholic Church has always taught that the Consecrated Host is the Body, Blood, Human Soul and Divinity of Jesus Christ. If you reject this, then you in total accord with those 1st Century Jews who said, “How can this man give us his flesh to eat?” and walked away.
Ultimately, this is a matter of Faith. We are not required to understand how this happens. We are required to believe that Christ meant what He said. When we all get to the White Throne we will not be tested on our understanding.
Why are we required to believe? Christ said, “Unless you eat the Flesh of the Son of Man and drink His Blood, you have no life in you.” So, do you want to be: alive or dead? There is no third alternative.
God bless
Transubstantiation, a later idea, is what I consider to be a rather convoluted attempt to reconcile the lack of literal evidence for consuming of body/blood with a more pagan and deeply personal and psycho-somo ritual of actual animal sacrifice and its consummation. By the time of John’s gospel, there is a real drive to separate believers from non-believers, and I think that John adds this more literal and “pagan” test as a separator to further identify the worthy from the unworthy. Unfortunately, IMO, this more extreme language later is latched upon and adds to the motivation to develop the idea of transubstantiation (literal body without the details of the body) which then becomes more difficult for some people to believe in as we move further and further from the mysteries of the past. It is one of the weaker “explanations” of Biblical language.
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This explains it all - “…it was soooooooooooooooo obviously meant metaphorically” - 
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think about it.