In the event of an Israeli attack, Washington would surely be accused of colluding with Jerusalem, severely damaging the United States' position in the region while provoking a ferocious Iranian response in Iraq, Afghanistan, Gaza, and southern Lebanon and the last thing the US needs is another war that may blow up in its face.
Another oft-overlooked aspect is the absence of public pressure in Israel for military intervention or of a supposed Iranian threat featuring as a priority issue for Israelis. The pressure to act is top-down, not bottom-up. And to the extent to which there is trepidation among the public, that is a function of fear at the blowback from Israeli military action, rather than fear of Iranian-initiated conflagration.
Finally, Israel's leadership is aware that its nonmembership in various nuclear accords and its assumed weapons-of-mass-destruction capacity will be dragged more harshly into the spotlight following an Israeli strike, not something that is likely to lead to precipitous Israeli disarmament, but unwanted, unpleasant, and unpredictable, nonetheless.