A wedding's location says something important about a couple, in what context they are pledging their undying love and who has a stake in the success of their marriage.
I think that most Roman Catholic dioceses in the United States have a regulation that the bishop's permission is needed for a wedding outside a church building.
Why? Marriage is a lifelong commitment, which the larger faith community has a responsibility to nurture. Linking weddings to buildings used by the faith community is one way of making that point.
Weddings are usually celebrated in church buildings for the same reason that Baptisms are celebrated there: That is where the faith community most often gathers.
That community certainly has a stake in the success of every marriage its members enter. Should problems arise in a marriage, will the husband and wife turn only to those who witnessed their exchange of vows?