You could be right but the fact that people go there indicate they want that convenience and cost savings. There is nothing preventing them from staying at Zorba’s place and having a do it yourself itinerary. I believe it is more families where dragging kids around is a pain who stay at these resorts. These types of resorts are popular at other places in Mexico, and the Caribbean.
It's like being on a cruise ship. The cruise might be cheap, but everything else is expensive and that's pure profit.