The first few questions were right  ..BUT the origonal real reason is  Back during WW2 the Union Tank Car Co. of Indiana Stopped making rail tank cars and started making turrets for tanks. All of the reject metal which was the holes cut out for the top of the tanks were saved because ou the cost of metal;. Now being the government CHEEP they saved the round cuts of metal. Some smart engineer seeing the giant mountian of metal discs said   ahh Man hole covers
Why are man hole covers round ?