Because they actually though he was speaking literally, as they were carnally minded, and consuming human flesh and blood is forbidden. But the contrast btwn the carnal and the spiritual is what John is always engaging it. Thus Nicodemus though "born again" meant physical birth, and the Lord did not spell it out to him what He meant.
But as in parables, the Lord spoke enigmatically so that true disciples would seek the meaning, and thus after the carnally minded left the Lord revealed that He would actually no longer be present in the flesh, but in Heaven, yet the words He spoke are spirit are life.
And unlike physically eating in order to have life within and eternal life, and live by Jesus, to gain life within, and possess eternal life by first believing the gospel is what is consistent with John and Scripture.
If not, and the RC literalism is true, then you MUST conclude that one must believe (in the Real Presence) and receive the Eucharist in order to have life in them, and eternal life,as perr Jn. 6:53,54.
But which is another example of RC private interpretation in trying to support a tradition of men. You need more study in exegesis.
It is not private interpretation but THE interpretation of the 1st, and 2nd and 3rd Century Christians who were much closer to the source than we are. It was THE interpretation for 1500 years.