Because shippers put themselves in a position where they have maximized efficiency and profit using extremely large and few in number ships. That means the loss of any one ship is a disaster for them and the insurer. If instead, the norm for the transit was to use smaller coastal freighters in larger numbers and then put the cargo on large ships for open ocean transit, this problem wouldn't exist.
Some shippers are seeing the risk of a passage as lower than the cost of a loss of the ship and having it sit waiting for the strait to become completely safe again.
Iran can claim whatever they want. It's being able to enforce that that counts. If shippers won't pay and Iran can't force them then Iran's threats are hollow.
What's your point? The Houthi made the same sort of threats about the Red Sea and were unable to make good on them there. I doubt Iran, using the same weapons and tactics, can do it to the Strait of Hormuz.