Why do people spend outrageous amounts of money for family affairs?

We've all been to outlandish weddings and other catered parties where people have spent thousands upon thousands of dollars for a one evening party. You know, the flowers the caterer, the hall, music etc... only to be over in a matter of hours. Yes it's lovely when you're a guest, but it seems so over the top to invite people you're so-so friends with and family your rarely see. I know it's a (once in a lifetime) thing but all included with favors, invitations... why don't these people just celebrate it on an extravagant vacation instead? I'd rather have that memory than a party that's over by midnight? What do you think?