The US and the Islamic Republic do not want the situation in the region to escalate into World War III. Both sides control the height of the flames, so as not to be dragged into a regional war.
Iran leaves the dirty work to the proxy organizations such as the "Islamic Resistance of Iraq" The two powers know that if they put the matter on the agenda, the other side might try next time to carry out an attack or terrorist attack of a larger scale. The second danger for Iran is its "face in front of the people"