Every neighborhood has a house like the Radley house that has reclusive residents who let the house fall into disrepair.
The Radley Place was inhabited by an unknown entity the mere description of whom was enough to make us behave for days on end …. (ch 1)
The key is the “unknown entity.” When the children are young, he is a monster.
Jem gave a reasonable description of Boo: Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that's why his hands were bloodstained- if you ate an animal raw, you could never wash the blood off. (ch 1)
Even some of the adults like to make up stories about Boo Radley. Some say he is dead, and others say he sneaks around at night peeking in windows. Most of the neighbors just feel sorry for the family.
As the children get older, they begin to be more curious about Boo. When Dill comes to spend the summer with them, he decides they should try to get Boo to come out. This embarks the children on a long journey of acting out the family story, trying to send notes to Boo, and otherwise obsessing with him. The presence of a new child to tell the story to breathes new life into it.
You have to admit, Boo's story is interesting. Imprisoned in his house since his late teens, Boo had an abusive father and an equally overbearing brother. He was guilty of nothing but normal teen hooliganism, but his family was rigid and reclusive. Boo never saw the light of day, had no friends, and likely only got entertainment from watching the kids in the neighborhood act out his very own life story.