It's a very simple reason, really: people treat him much better when they can't see for sure what he is. Or at least, that's been Shrike's experience so far in his life. And that fear goes double when there's a chance of rejection from someone who's already treated him kindly, as Shrike's known so few of those people he'd hate to lose good standing with any of them.

"I don't know..."

Still, he's not going to argue his point. Arguing just makes things worse, in his experience.