It should be obvious that, even at the Last Supper, that Jesus was speaking symbolically as he held up the bread, broke it and handed it out as well as the shared cup of wine. The bread and wine did NOT change physically, but remained the same as they started.Information on Eucharistic Miracles: http://www.therealpresence.org/eucharst/mir/a3.html
It would be easier for the world to survive without the sun than to do so without the Holy Mass. --Padre Pio
Exceptions don't prove the rule. Any number of explanations can be given for what supposedly happened in those few incidences, but it should always be happening if it were really true that, when the priest consecrates the bread and wine, they truly change into the real human flesh and blood of Jesus. If one were being honest, she/he would admit that they don't change in any observable way and that it MUST mean a spiritual representation. Even when Jesus first initiated the observance, he spoke symbolically and he was understood that way.
It was only centuries later that those who thought some sort of miracle MUST be going on in order for the "sacrament" to be a measure of grace decided to develop the doctrine of transubstantiation. It wasn't until the Fourth Lateran Council in 1215, that the Catholic Church formally adopted the doctrine of transubstantiation as its official teaching. This was confirmed by the Council of Trent (1545), which also asserted that the Lords Supper was a propitiatory sacrifice for sin.
If you would like more information about the development of the doctrines and the writings of early church theologians concerning it, please see The Eucharist.