When discussing science, a lot of people confuse why with how questions. Why is the sky blue? Who knows? It just is. But how is it blue? That's been answered. To your point, I think you're right about why being not the best, or even relevant, question to ask. In school, I teach my students to ask why questions about characters motivations. But you can tease those out from close reading of a text. Not so from a living person, who, like you say, will not usually incriminate themselves. Why do shooters kill children? No one knows. How the hell did he get the gun? Who gave it to him? Did authorities know about him? When did they know? The Why question never gets answered, and it wouldn't stop future shooters if it did.