Because as soon as they see our missile launches, they will launch at our cities?
Let's say Iran is secretly supplied with new Chinese or Russian long-range anti-ship missiles. We go into the Persian Gulf with a Carrier Battle Group, with another two CBGs in the Indian Ocean on standby. The Iranians launch a swarm of missiles that sink all three carriers, and most of their support ships, wiping out over a quarter of our carrier assets.
Now, are you going to nuke Tehran and Qom in response, killing millions?
That is a definite YES.
The only issues I’d have would be “How much fallout and where will it end up?”
I am sure as hell not going to risk lives of more American service members to put paid to the savages. They don’t deserve that sort of consideration. (Wish we’d nuked Kabul and Kandahar - that would have quieted the ME down...).
And millions of dead Iranians would be a good balance to thousands (tens of thousands?) of dead Americans murdered in a sneak attack. Read “The Grave of Hundred Head” sometime.