Taboos are never iron laws, of course, but the world's superpower loudly and repeatedly rejecting them both in theory and in practice does have consequences.
The war has taken a personal toll on the president and some of his senior advisers. Some top aides have been advised by intelligence agencies that the Iranians are seeking to kill them and have been told to stop taking car services, according to people familiar with the matter.
Arguably pulling a "Venezeula" is a more moral course of action (not that it is moral) than bombings and ground invasions which kill or remove everybody but the leaders, but you can understand why self-interested world leaders have tended to take that option off the table.