Why does the Catholic Church have "closed communion?"

About 150 AD Justin Martyr wrote in his first apology:  "And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins, and unto regeneration, and who is so living as Christ has enjoined." 

This is what may be referred to (by some) as "closed communion." If your unbaptized relative comes to Mass, they cannot receive. If your non-practicing Catholic child comes to Mass with you, they cannot receive. If you have not confessed serious sin, you cannot receive. If you do not follow the teaching of the Church, and believe that the Holy Eucharist is "not as common bread and common drink" but "the flesh and blood of that Jesus who was made flesh," you cannot receive. 

Why? 

We have come to call the Eucharist "Holy Communion," and for good reason. As a sacrament, Holy Communion makes present the graces it signifies. When it comes to actually receiving the Holy Eucharist, Jesus is really and truly present, there is no symbol there. The symbol (at this point) is the communion that occurs between Christ and the individual and the individuals together in Christ as Church. Holy Communion signifies this, but it also makes it present. 

In other words, you cannot separate the act of receiving from the rest of the Mass. If you cannot live the Mass fully - from the Confiteor through the great Amen - you cannot honestly receive Our Lord. It is not a judgment against individuals, but a reality that we respect. 

What sense, even just logically speaking, does it make for a person to approach the King of Kings with open hands and closed heart.  They relive the recent Gospel and seek only a bread king, and never look to the Sacrifice of the Mass on Calvary. 

 It is not Catholics that have "closed communion" then, but rather individuals that are closed to communion. We, as receivers of the great Gift, must reverence the powerful presence of God in the Holy Eucharist. We do not wish that he go knocking where the door is barred shut. Instead, we prepare ourselves to receive him, praying especially to the Mother of the Eucharist. And, we strive through prayer, example, and education to help others to unlock the door of their hearts, believe, and approach the Eucharistic banquet.