The United States has been tracking, with more success, these shipments as they leave Iran and alerting countries whose coastal waters these ships enter. This has led to several interceptions and seizure of large quantities of weapons. The Iranians have taken to hiding the weapons on these ships, but that is not a perfect solution on a ship, where anything hidden is found eventually if the searchers are sufficiently diligent.
Iran denies it is involved in this arms smuggling and just keeps at it. In addition to supporting rebels and Islamic terrorist groups it favors, the arms are often for sale, not free. The Iranians want to be paid in dollars or euros and this money pays for needed items that have to be smuggled back into Iran because of the many trade embargoes against Iran (because of their nuclear weapons program and support of terrorism). The main recipients of these weapons are Shia tribal rebels and al Qaeda in Yemen, al Shabaab and al Qaeda in Somalia, Hamas in Gaza, and the government of Sudan. Many other arms dealers in the region are also customers.