If it's a larger event, the purse for winning easily covers the hole-in-one celebrations. On top of this, if you are a member of a local country club - you're already paying hundreds if not thousands a month on membership dues. You're expected to have such wealth that to cover drinks/food isn't a big deal. Some clubs are structured such that you aren't directly paying for the celebrations, but your membership dues are such that it includes the cost of any potential celebrations.
Golfing is a game for the wealthy, including the traditions surrounding it.