It really isn't. A huge portion of Iran's air defenses are designed for road-mobility and pop-up attacks instead of long-term point defense, encompassing hundreds of launchers total: https://en.wikipedia.org/wiki/List_of_equipment_of_the_Islam...

Military strategists long warned that air campaigns flying over South Iran would have to contend with passively-guided SAMs and MANPADS on their way to Tehran. There are hundreds of road-accessible caves in the Zagros range that cannot be inspected via satellite. They inherently present a risk to overflights unless they are occupied on the ground first; it's common knowledge why Kohgiluyeh and Fars are so dangerous.