This may be the case in some parts of the UK, but it is definitely not the case in Southern Hemisphere countries.
Firstly, no one here would have a BBQ (real term - braai) with burgers. Burgers do not go on BBQs here. There is always a variation of meat.. chops, steak, wors (sausage, but not the banger kind), sosaties (kebabs), pork rashers,,ribs etc. Most people cannot afford to supply all of that for everyone, so it is an unwritten rule. You get told ... braai, my place, Saturday for the rugby.... There is no more discussion. You know to take your cooler boxes, one is filled with your drinks for the day/night and other contains the meat. No time is given, you get there before the rugby starts. Even though the host will supply salads etc, there may be a discussion of shall we bring that pasta salad everyone raved about last time, and hey do you guys have enough rolls.... and then in the cooler box that has space, a few bags of crisps will be chucked in, and probably some dips..... as we all know there is no way we are eating until AFTER the rugby.
We entertain outdoors a lot, all year really. So this... you who hosts must supply everything would not work here at all. We pick up the costs for ourselves for the meal, and the next weekend someone else will host and everyone does the same again. The food all gets put together and cooked together, so if you brought fillet and get rump, well its no biggie (to be fair no one is taking fillet tbh). And the person hosting usually gets the keep the leftovers, so they have a great breakfast and lunch the next day.
It is all very casual. Some of you seem to make out its a real formal affair, invites, eating at certain times, all food should be supplied. Can I suggest you make friends with a South African, so you can pull in for the rugby and then you will see. 😄