Receiving the Eucharist is not cannibalistic because we are receiving the Glorified Body & Blood of Christ. If, for example, the 1st century followers had heard Jesus words “eat my body… drink my blood” and then actually eaten him after the crucifixion (ridiculous, I know, but hypothetically…) they would not be receiving what we receive in the Eucharist. That was the physical, earthly body of Christ ONLY. What we receive in the Eucharist IS the physical Body and Blood, but also the Soul and Divinity of Christ’s glorified Body. I hope that makes some sense.
The OP also raises an interesting question as to whether there is something inherently wrong with cannibalism. Certainly if a person was killed (even if they consented) for the purpose of being eaten, that would be murder. However, in the case of say, the Donner Party, who were forced by starvation to eat members of their party that had already died, was that wrong? It does seem disrespectful to the dead person, and may be an offense (if not a directly intended one) against belief in the resurrection of the dead. (Sort of like how the Church says you are allowed to ask for cremation after you’re death, but only if it’s NOT for the reason that you don’t believe in the resurrection of the body).
All that aside, though, the thing that I think might make cannibalism inherently disordered is that when you consume the blood of something, you absorb it’s essence. (I’m not sure how that works). It’s the reason the Jews for example would completely drain the blood from animals they were going to eat. They never consumed blood, because God’s OT laws said not to. The reason we drink the Blood of Christ at Mass is because we are absorbing the essence of Christ himself through receiving him into our bodies. This is what gives us all the indescribable benefits of Communion.
Anyway, maybe I’m just rambling, but it seems like consuming someone else’s body would be wrong because you’re absorbing some part of who they are. Their tissue is assimilated and becomes part of you.
I welcome someone more educated to fill in the large gaps in my understanding of this question.