Most people use sites like Expedia or Kayak to compare options because they don’t start off knowing they want to fly American Airlines or United. They find the schedule and price they’re looking for, and then book what looks best.
While it can be useful for the average consumer to search flights with Expedia or the like, it’s not actually a good idea to buy travel from them unless they are able to construct complicated itineraries at a lower price than booking directly. These two tweets show why.