Technically, there is a clause in Church law that could allow for Catholic weddings to take place outside of a church, in a suitable space under special circumstances, but it requires the rare persmission of a bishop. 

But the meaning of marriage goes very, very deep, and the Church has a vested interest in protecting that meaning. As Catholics, we don't see a wedding as an event that needs a pleasing backdrop. 

Marriage is a Sacrament, an occasion of joyful promise and symbolic solemnity. It is a spiritual event. The loving bond of marriage symbolizes the mystical union between Christ and his Church. The man and woman profess lifelong vows of fidelity, and they receive supernatural grace from God to keep their promises.

Receiving the Sacrament of marriage in a church has broader symbolic meaning for the couple as well. Marriage isn't simply two people in isolation from the world around them. Marriage is the joining of families and the beginning of a new, holy mission in the life of the couple. Weddings are sacred occasions, and it is fitting they occur in sacred spaces.