I remember how well "shock and awe" went.
No one is afraid of a conventional war with Iran. However, there is almost no chance that a war with Iran will be conventional. The weapons they possess that are the most formidable are of the cyber variety. Sure, we can bomb the hell out of them, but all of that is superficial damage. On the other hand, they've been working for over a decade to infiltrate US cyber networks. It's possible they've been completely unsuccessful and we have nothing to worry about. It's also possible they can shut down our power grid, banking systems, and/or wireless communications networks at a moment's notice, causing chaos for US civilians and military alike. And in an America in chaos, it would be very easy for Iran to deploy another weapon they have plenty of experience wielding - terrorism.
That, of course, is assuming that a war with Iran will be with Iran alone. Russia has issued statements condemning the killing of Soleimani as a criminal act, and is an ally of Iran. Remember how the Assad regime was toppled in Syria? Of course you don't, because it didn't happen; Russia intervened and made victory in Syria for the US-led coalition an endeavor that was not worth the cost. I have no doubt that they would do this again in Iran, and the US has no allies in this fight.
Then there is the question of what happens after a US victory. Does the US stay and work to stabilize the nation, at the cost of thousands of US military lives and trillions of US tax dollars? Or do we burn the place down and walk away from the wreckage, leaving the place a hotbed for terrorist activity and a constant reminder that furthers anti-American sentiment in the region, and requiring countless covert missions to stem the tide of terrorism? Both of these options commits the US to continued military activity in Iran for years (the former in a similar way to our presence in Iraq but on a much larger scale and with no help from other nations, the latter similar to Afghanistan but without a government in place to cooperate with), in addition to continued involvement in Iraq and Afghanistan.
War is not even remotely as simple as what you describe. It never has been, and is increasingly less so.