It’s not just a cost thing when people don’t turn up at the last minute though. It’s more a matter of ‘oh, we thought enough of you to invite you, and then you just fucked it off’. Could have invited someone else, or spent less cocking money, etc.
At our wedding we had a hog roast so cost per head wasn’t really an issue. What we didn’t want was loads of people we didn’t even know at our wedding (as it is the best man and one of the ushers brought girls they’d been seeing for a few months, who we’d never met before and haven’t seen since, but are in loads of our photos).
Luckily she turned out to be awful and we’re not married any more, but you can see why people don’t want random folk at their big day.