I noticed that if I dropped a kitten onto a glass table from about 2 inches, he will bump his nose because he doesn't see the glass and isn't prepared to take the impact. So maybe they're not used to glass and don't know for sure they can walk on it.
You owners with cats who walk on the glass, are those glass tables perfectly clear or are they smoked or colored? How long did it take them to walk on them? Did they take a while to figure it out or did they just jump on right away?
I had a cat who was facinated by glass, and my glasses. She thought it was neat to have a substance that you could see through but not move through.
I've also had cats afraid of mirrors laid on the floor. They think it's a hole in the floor with a big room under it.