Because the effective transmission is by consuming it and making Him a part of you. Not looking at it or praying to it in a gold box or in an Aztec inspired monstrance. Descirbe the sacramental effect of an unconsumed host? If there is none, you have created an object of worshiip, an idol. Some parishes have figured this out and put the tabernacle in a separate room, including the parish I was baptized in. I was miffed at first, because I remember when they switched from the Tridentine to the New Order when I was child, but then I thought about it and it made sense. I still say the usual prayers in preparation for Communion, including some that really bring the Passion to life, for example, Eloi, Eloi lama sabacthani at the first consecration and I thirst at the second, but that is for me, not for God. God wants me in Communion.