St. Thomas Aquinas
“It is evident to sense that all the accidents of the bread and wine remain after the consecration. And this is reasonably done by Divine providence. First of all, because it is not customary, but horrible, for men to eat human flesh, and to drink blood. And therefore Christ’s flesh and blood are set before us to be partaken of under the species of those things which are the more commonly used by men, namely, bread and wine. Secondly, lest this sacrament might be derided by unbelievers, if we were to eat our Lord under His own species. Thirdly, that while we receive our Lord’s body and blood invisibly, this may redound to the merit of faith.”
Source: St. Thomas Aquinas, Summa Theologica