Why are nodules not mined for metals such as manganese and iron?

Answer 1

Because they only exist on the bottom of the oceans and no tools have ever been developed to recover them from that depth economically.

The one attempt to "mine" them that was heavily covered in the news worldwide was actually not doing that at all and was not equipped to do that. It was actually a Top Secret CIA mission coordinated with Howard Hughes' companies to attempt to salvage intact a Soviet nuclear submarine and all its equipment (including missiles and torpedos) that had sunk in an accident. The salvage attempt was a total failure with the capture claw intended to pick up the submarine breaking followed by the submarine breaking in three parts. The two larger parts with all the things that the CIA wanted sunk instantly to the bottom and the one remaining small part that was recovered contained little more than the dead bodies of the seamen. These bodies were buried at sea. No truly usable intelligence was obtained from the mission.

At the time the cover story that they were going to mine nodules triggered a lot of research by various companies on how to do it so they could try to mine them also. It also triggered a significant backlash from the environmentalist movement raising concerns about unknown damage to undersea ecosystems. But the entire thing died out soon after the special ship returned and was immediately put in the mothball fleet in Suisun Bay. If Howard Hughes could not do it, nobody else could (but nobody know he had not even tried but was doing a Top Secret mission for the CIA instead).