The idea of the eucharistic meal comes from the New Testament Gospels, where Jesus has a meal with his disciples before he is executed.
Jesus says to break bread and drink wine in memory of him. The bread stands for his body, the wine for his blood – he was about to be executed by crucifixion.
So, create a ritual of a meal where the bread and wine stand for what happens to Jesus, so as to remember the events and him.
Pretty straightforward, right? Not when theologians get their hands on something. The basic error of theology is in linking together lines of reasoning (even if any one line seems arguable or reasonable) without an ability to robustly test the claims.
In Catholicism, what was a straightforward, completely understandable, and not very contentious idea (that by having a ritual meal, you can bring into one’s thoughts and therefore actions, Jesus’ teachings and actions) becomes something that seems almost un-understandable – that the ‘essence’ (something that cannot be tested through conventional empirical means) of the bread and wine is ‘transubstantiated’, and that this can only be done by a blessing of a specialized priestly class.
There are lots of cases in contemporary Christianity where it seems theologians have been given too much reign.