There are other factors too. The US has a limited number of bases in the region which Iran is systematically taking apart right now. These provide logistics for staging, ammo storage, early warning systems, air defenses, and so on. These bases took decades and trillions to build, and they simply cannot be replaced. While Iran is simply too big to bomb into submission, the same logic doesn't apply to the limited US assets in the region.

Another huge factor is that the US has to ship weapons across the ocean, which is slow and expensive. Iranians are fighting on the home turf, and they effectively have unlimited weapons by comparison. And of course there's the fact that Iranian weapons are simply more efficient. The US is forced to use multiple multi million dollar interceptors to take down each drone that costs a few thousand to make. It's obvious that Iran can pump out drones faster than the US can pump out missiles.

Finally, the whole idea of ground invasion is a fantasy. Iran is a country of 90 million, with difficult mountainous terrain. For context, Ukraine is a third of the size, and Russia has an army of 1.5 million fighting there.