I think that they have a cross between two personality traits
1) the airport bum. This is one I have. I like airplanes for some reason, and whenever I have the chance I enjoy watching them....watching takeoffs and landings, etc...
2) the collector. Some folks just like to collect things. I have suffered from this myself in some ways. One that comes to mind is certifications. When I was younger and actually had free time, I collected certifications. Mainly just for something to do, to give me purpose. I got my pilot's license, then my instrument rating, then I worked on a multi engine rating even though I'll likely never own a twin or need it, then I worked on tail wheel sign off, and ant one point even worked on a commercial...again for no good reason. For SCUBA diving, I collected a whole litany of C-Cards...open water, advanced open water, a few "specialty cards" , nitrox, dive master, assistant instructor, technical diver, deep diver, trimix diver, gas blender, etc.... Not that was ever planning to work in the industry only because I was diving anyway, so might as well work on something.
Seems to me that these planes spotter fall into that mode...they're being airport bums, might as well record and document how many different aircraft they have "seen"
How about the dudes that barge into and all around people's aircraft camping spots in the North 40 at Oshkosh? They like to flit around the N40 during the airshow. They get a little too close snapping pics of your plane, scribbling in their notebooks, and generally don't even say hi. I'm sitting under my wing enjoying a beer and the airshow and they act like you're not even there. They push my Midwest Nice limits.
I'll admit to catching myself doing this sort of thing. I think in my case it's because they are fixated in airshow mode looking at the machines.... + there's a real chance that you could just be some random other spectator hanging out near this cool plane too, just like me. No way to really know if you're not paying attention, that YOU are the owner.