I'm a lifelong spotter, so is my wife. Few things I enjoyed better than when my wife drove the kids up to Heathrow and we'd meet up after my morning watch and enjoy an hour or two on the Queen's Building. Sadly, it is no more. When I first used to visit, back in the 50s and 60s, the place would be full of spotters and Joe Public because aviation was all "the thing" and everyone wanted to watch aircraft. Unfortunately, diminishing interest made the roof gardens too costly to run for fun, plus the possibility of terrorist activities became very real.
Real estate on a major airfield is worth gold and there are plenty of commercial outfits willing to fork out the money for a piece. The income from spotters compared to the cost of providing facilities would be tiny. For Heathrow, there are a good few places off-airport where you can watch aircraft in total freedom.
I know other major airfields do provide for spotters and there are countless smaller places (eg Popham) where spotters have free access.
Just accept that things are the way they are. We derive as much fun sitting in the garden on a clear day doing our spotting with the help of our binoculars and SBS.