X-rays would do better, but you know, dangerous. On a less sarcastic note, yes, there are frequencies which would be better suited for a specific target materia

X-rays would do better, but you know, dangerous. On a less sarcastic note, yes, there are frequencies which would be better suited for a specific target material, but like Jach said, regulations and cost are what keeps this things in the WiFi spectrum. I think it also has a kind of "wow factor" when told to non-technical people.