Good question! I'd say because they have evolved from the earliest coins which were bashed out of hot precious metals by a blacksmith on an anvil - and imprinted with an insignia. Hundreds of years ago it would have been very hard to produce any shape other than a disc-like shape but if you look at Roman or Greek coins they are far from perfectly round anyway! Because the coins were made of precious metals which made up their value, people were tempted to shave bits off, melt them down and make new coins - this is a form of debasement, and on a large scale would result in a devaluing of the currency so that it would eventually need to be recalled. And if you start with something pretty much circular, it's more difficult to shave some off I guess! Plus, coins pass through a lot of hands, so would get worn down anyway, so a square or triangular coin wouldn't have sayed that way for very long.