i think bride to be really prepare this occasion as the highlight of their lives. they wanted it to be the most memorable day in their lifetime. especially if this is their first time getting married and thinking they want to be the perfect queen for that day. then, even its the most expensive wedding dress there is, they keep it as a souvenir, a memento of their happy young life, starting together.
some bride expect their groom to be well prepared as well, not just like a rental tuxedo that they can return, some bride, keep the two clothing to preserve for their children and grandchildren to see (if the marriage will last that long).
those clothing can be wear again if their size were still the same after several long years, think about the silver & golden wedding anniversary, they do still tie the knot accordingly. what would be the best way if you can still wear the same one? if not, alteration is the other option.
plus if they pay that much, they will be the center of attention comparing price from their other married friends...