Why is structural beam width always 9 inches?

conventionally the brick work done underneath of the beam is 230mm or 115 mm thick.every customer prefer no offset should be visible in the room.in case of 230 m.e after the plaster completed no offset shall be seen from the room.due to thus reason only the beam thickness made as 230 mm.now a days it changed as 200 mm tho as sac blocks is in use.