Because it is simple restaurant business fact that self-serve buffets guests put more food on their plates and waste more food vs other types of service. I used to work in the hospitality business and have friends that still do and they have even commented since they changed to cafeteria service ( covid reason ) there is much less food waste. The problem is depending on what market you are in human costs are more than food costs. So it’s cheaper to waste food versus higher staff. Some markets food is more expensive ( 5 star hotel in Asia that has to import quality western food) vs staff.
Edit: I vaguely remember a study that dealt on this topic during school. I believe in the buffet they had like five different types of pizza slices. Self-serve people will take three or four different slices because they were thinking to themselves oh I can try and see which one I like best and then get more. If they had to ask for what pizza they wanted they reduced the number of choices. I believe the study wasn’t really focusing on portion sizes or how much people eat but more on how a consumer deals with choices . Self service is considered “free “meaning I can take all and what I want. And having someone serve it to you it’s more like asking not taking.
