Why? I think our islands are even more vulnerable because once they're invaded, we'll have no way of moving our troops there. Taking them back will be very difficult without an air force and a navy.
The mainland Iran is harder to invade because we have plenty of ways to get there, and we can even use Basij and regional militias to fight there. We cannot do that on our islands.
Assuming they invade an island of ours, they will provide them with air-defense and close air support and they will relocate to strategic places there and hide in our buildings. If we hit our islands, we won't kill only US troops, we will destroy our own islands as well.
Plus, you're comparing the cost of a bullet which is less than 50 cents for us to short-range ballistic missiles that is probably about $100K, or even a suicide drone which costs like $30K.