It’s obviously a big deal, but it’s true there’s no logical reason to invite so many people and spend so much money. (The only possible reason I can think of is to discourage divorce or calling it off, because of the big investment.) It’s only custom for it to be such a big and expensive party.
Really, people should just invite 20-30 of their closest friends and family, cook basic meals for the guests themselves, and have the event somewhere cheap or free, like a park or someone’s backyard. There’s no reason to invite people to weddings if they’re a distant relative or barely know the couple.