The problem is that you can never be sure where a 'hack attack' came from. Sure, the directly used IPs are from Iran, but it is very well possible they were simply used as proxy. Most of the world already regards Iran as evil, so they are a good scapegoat. Obviously it's possible they are really behind it, but you can't be sure enough to start a war.
Let me put it this way: if Iran has deniably disabled US nuclear capability in the same way that Israel deniably disabled Iranian nuclear capability, the spectrum of options considered by the US government would certainly not end with "send them a strongly worded letter, since we're not totally convinced they did it."
I mean, another country in the neighborhood went for strategic ambiguity for many years. It worked very well right until it didn't.
I certainly don't disagree that it is very dangerous and might cause countries to attack each other in extreme cases. It does give a third party a relatively cheap strategic option to pit countries against each other.
You make it out like definitive proof is necessary for escalation. Don't forget the Lusitania, the battleship Maine, the Reichstag Fire. All of them had ambiguous causes at the time but severely escalated violence.
