The reason you find 'ghost tours' boring when in a large group is because of the safety in numbers instinct.
If you are predisposed to believing in 'ghosts', revisting a site that is supposedly 'haunted' when by yourself or in a smaller group calls into effects the "mass histeria" effect. Example: 1 person looks at the sky, person walking past sees, looks up. So on, and so forth.
I personally don't believe in 'ghosts'. Tunnels are quite effective at causing an increase in reverberation of sounds and the wind across the entrance of tunnels has a 'conversation' effect sometimes. Old houses creak, moan through winter and summer changes. Eyes adjusting to variant light causes false shadows and the human brain creates false homosapian shapes quite effectively.
In summary, if ghosts and the creepy noises did exist. They should be seen and heard whether in a group of 100 or 2 by fact a lone, unfortunately, they don't and it's just your mind playing tricks on you basing it's perceived environment off hollywoods version of ghouls, ghosts and creepy things that go bump in the night.
What scares me? Julia Gillard.