You know why LEDs are square? Because it's easy to cut them from the wafer that way. Try cutting a circle with a circular saw, not easy to do, that's what dicing saws are. Also, with square die they all touch each other and you just cut little lines to separate them. With a circle you have 4 infinitely small points that touch adjoining die with an unusable cross shape of material between the 4 adjoining die that is useless, this means waste. So to sum up, cutting circular die requires special tools, it wastes material and will thus be more expensive.