Being brutally honest with yourself, can you ever see yourself answering the question "So what are you doing this weekend?" with "thought I'd hang out at Heathrow. It's got a superb chilled atmosphere and there's always something interesting to do."
Once upon a time, people did go to the local airport for its social/amenity/leisure value; now most terminals have become little more than glorified bus-stops, expensive to use and maintain. Customers/passengers don't want to spend any longer there than absolutely necessary and airport managers rejoice in statistics that show how many hundreds of thousands of people pass through their facility
Yes, back in the day we
did go on to the airport for social reasons (most couldn't afford to fly!), to spend time on the viewing terraces, a bit of people-watching and celebrity spotting, or to see mates who worked in the landside, etc.. After the pubs closed (2230 or 2300 in those days), it was off for a late night cup of coffee, either at Heathrow or Heston services.
Of course it couldn't happen now because of security considerations.