Some of those "lurker's" could be young folks who just want to see what us old/older folks are talking about/saying. They would have no intention of joining a Senior website, because they know it is a website for Senior age people. IOW, some "lurker's" are just plain curious, but don't/won't join in.
I think there have been several Threads wondering about "lurker's". Apparently, the "lurker" thing really does disturb some of us. Doesn't bother me........I've got use to seeing a high number of people "looking" at mine, and other's, Threads and not replying. Just the way it is.
Same thing goes with filling out the Profile page. SB says it's not a requirement, but many members of this forum sure would like to see it done more. Thing is, not everyone wants to be open about themselves as some others do, and since it's not a forum requirement, some "newbie's" fill in very little, if anything at all. Again..........just the way it is.