Conversation

"While there exists a classic
body of literature for the electromagnetic properties of various materials, the actual reflective properties of typical
external building structures at microwave frequency bands are
just beginning to be explored."

From a 1996 research paper.

Only 22 years ago, they didn't really know much about how building materials reflect microwaves. Microwave links aren't exactly new, but indoor microwave transmitters (WiFi) didn't exist, so I guess that makes sense...