It might not be. It might be. Uncertainty is the point of what Iran is doing.

There might be mines in the straight that are sophisticated enough to be armed, disarmed, or moved on command, or there might not. There might be artillery emplacements* hidden and not found, ready to pop up... or there might not. There are probably still plenty of drones and missiles all over the country that can be called down on Hormuz at will. Iran might choose to save them for something else... or they might not.

If a few oil tankers get through without Iran's permission, one might conclude everything Iran has in place has been found and that the straight is safe. Then again, it might not be. The Iranians might save a few choice surprises for the first aircraft carrier that gets too close. They might also choose to actually sink a large ship**, blocking the straight long-term. The Iranian regime has been planning specifically for a U.S. invasion since it's inception*** and they probably have some very well hidden and nasty surprises as well as plans to use them to maximum effect.

Merchant vessels can't get insurance to go through because of all this uncertainty. The U.S. Navy has completely refused to go in there because losing a multi-billion dollar military vessel along with hundreds or thousands of sailors for a war that's already unpopular would likely knock the U.S. out of it completely. This is why Trump is desperate for other nations to come in and clear the straight. He doesn't care if they lose ships, but he can't afford to lose even one American ship for a "Wag the Dog" war that's already exploded the budget.

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*The straight is narrow enough that artillery can actually cover it. Even the most sophisticated anti-missile defence systems aren't meant to deal with artillery shells fired from nearly point blank range.

**The straight has only a couple of channels deep enough for large vessels to transit. One or two well positioned wrecks could block the works.

*** They rebelled against a Shah installed by a CIA backed coup after all.