Short answer: because it’s austere. Long answer: wedding site builders has many features aimed towards nubile couples that simply outclass the poor service of Facebook. First of all, your site should have the same tone and visual as the ceremony. With the huge gallery of designs, your guests would feel much more motivated to honor the event. There are also management features that allow you to sort the RSVP answers and see who visited your page. This just the tip of the iceberg though, for a more detailed list, check out the Features section.