Why do you think a boxing RING is SQUARE?

Initially, it would have been a ring but a square would serve much interest of the manufacturers of the arena as it is always easy to build a square structure than a circular structure. Also the seating arrangements and everything will have to match the circle. In a small boxing ring, it would not be cost effective but for a stadium, it will be cost effective too.

A word of Caution - Above statement is author's own perception and prejudice.