Metal light switches and lights etc MUST be earthed. I guess you know why. Under fault conditions, metal could become live.
"Earth" is a misnomer. In simple terms, it doesn't necessarily mean the "ground we stand on". (Even if your supply is a TT system with an earth rod, there may be far too high a resistance between your switch and the earth rod for a fuse/MCB/RCD to operate in a fault.)
Other, more common, supply systems have either an "earth" brought into the house (separately) by the supplier........... or it may be one that uses the incoming phase neutral as an earth. So, you can see that your switch would be a long way from, and not connected to, the system's earth.
In order to try and clarify things, new rules now call the green & yellow wire around your house the CPC (circuit protective conductor). Although it's connected to it at the board, it's a completely different thing from your incoming earthing arrangement.