@Catsandcheese It's not pay as you go, it's a gift.
You are highlighting what the problem with cash is - expectations.
Some people have expectations that guests will pay for themselves.
That never happened with a gift.
It costs a fortune to attend a wedding.
Outfits, travel, childcare (if childfree) accommodation, hair, loss of earnings (for some), hen do/stag do costs.
So if you're looking at a transactional thing - I would say if the costs of food/wedding for a family of 5 is £300-500, then I would say that is less than the costs above. So no gift needed - and yet people still offer one.
That's because a gift is by it's name something you choose to give, to celebrate the event, not to repay a debt.