I'm not an expert on shopping malls, but the economic argument for the notion of a shopping mall as a platform is the agenda of the mall operators. Their game is to bring in lots of merchants and lots of shoppers and provide this platform. The second part of their business model is to then extract money. These are businesses. They want to make as much money as possible. That's their job.
The interesting part is who they extract the money from. What you don't see is that when shoppers walk into the mall, shoppers have to pay to get into the mall. What you have is the shopper coming in free, but the merchants and the shops in the mall are paying for essentially all the facilities that the mall provides.