In to Kill a Mockingbird, why does Miss Maudie say Scout is being morbid?

Scout starts hanging out with Miss Maudie without Jem and Dill when the two started plotting in the treehouse and generally acting like they didn't want her around (calling her a "gi-irl"). Scout asks her one day if she thinks Boo Radley is still alive. Miss Maudie says, "His name's Arthur and he's still alive." Scout asks her how she knows, which is when Miss Maudie said, "What a morbid question" (48). 

Morbid means "having an unhealthy interest in disturbing and unpleasant subjects, especially death and disease." It's a morbid question because Arthur "Boo" Radley hasn't been seen outside of his house in more than fifteen years, and there are multiple neighborhood ideas of what became of him. They have made him into a "malevolent phantom," in their minds, anyway. It's a bit morbid to imagine that Arthur is in the house but long dead, which is what makes this a morbid question.