Because they thought Christ was talking about literally eating His flesh and drinking His Blood-which He wasn't!
He was speaking of faith and using 'eating' as a metaphor.
57 As the living Father hath sent me, and I live by the Father: so he that eateth me, even he shall live by me. 63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life.
Matthew 4: 4 But he answered and said, It is written, Man shall not live by bread alone, but by every word that proceedeth out of the mouth of God. Dt 8:3 Luke 4:4
Excellent.
You can call Christ a liar if you wish, if that is your personal interpretation, but you do it at your own peril.