Probably since everyone started wearing headphones. In my opinion, thanking the driver is a courtesy, but not required. I usually just give them a wave as I'm leaving the back door. If I was a driver, I think I'd get sick of having to acknowledge every thank you. But that's me.

The one that annoys me is people stopping in the middle of paths/walkways/aisles, usually in the dead middle, to have a chat with zero regards towards anyone around them trying to get past. Just step to the side, pretty easy to do.