It is hard to see any American interest at all in Iran, and yet four presidents have now expended an immense amount of political capital on the country.
The answer to this question is actually simple. It is in Israel's interest for the U.S. to treat Iran as a supposed "existential" threat to the world. Having the U.S. so engaged in Israel's cold war with Iran serves Israel by keeping the world's attention on a supposed dire threat to world peace instead of on the Israeli apartheid system for Palestinians. And it serves a master Israeli politician Benjamin Netanyahu in his effort to stay in power and out of jail by manufacturing one enemy after another and bragging of his closeness to an American president.