This saves on cash, encourages creative volunteerism, and gives encouragement to people to take a pro-active role in the bridal shower. Some of the finest food was born out of forced volunteerism, as it were.
Bridal shower invites are commonly cheaper than other cards as the event isn't all that significant. It isn't as major as Christmas, a 50 year marriage anniversary, Easter, or a birthday. It essentially sets the tone for the newlyweds content ending.