It's an interesting question. Not a spotter myself, but I think interest in trains and aeroplanes in particular is a bit of a hang-on from a more "romantic" era (circa 50s/60s).
The drama of some big steam engined mechanical beast roaring past in a cloud of white smoke would have been entertainment to many before the electronic age. And aeroplanes at the time represented a sense of glamour, adventure, and technological advancement.... A silver bullet taking flight to the edge of space, enroute to shores far away, flown by some ex RAF WW2 hero.