Why did Dill run away? What do Jem and Scout think stops Boo from doing so, too?

Dill ran away because no one aid any attention to him. He felt isolated and alone, so he left for the Finches where he always felt loved and included.

“That wasn’t it, he—they just wasn’t interested in me.”
This was the weirdest reason for flight I had ever heard. “How come?”
“Well, they stayed gone all the time, and when they were home, even, they’d get off in a room by themselves.”
“What’d they do in there?”
“Nothin‘, just sittin’ and readin‘—but they didn’t want me with ’em.”

The children surmise that Boo doesn't run away because he "doesn't have anywhere to run off to..."