I saw a hollowed out US half dollar in this reddit post, and thought "I wish all my coins were like that".
Doing a quick search around, most coinage seems to be quite solid. There seem to be some around, like a few of the Japanese yen coins with a hole right in the middle, but none to quite this degree.
I'm far from an expert on these things, but it would seem that making mostly-hollow coins would greatly decrease the amount of material required, as well as lightening the physical weight a lot of people's pockets.
Why aren't there more hollow coins around?