Because those serial numbers would be long as hell. Have you seen how many cents the US mints every year? Where you gonna put a ten digit number on a coin?
The edge? Those would be some mighty small numbers. lol They seem to be able to put the year, mintmark, some stars, and e pluribus unum on the edge of a dollar coin so.........
Not to mention how are you going to get something that can take the force to impress the number into the metal and then change for the next coin and do it again in less than a fifteenth of a second?
