If you want a real explanation, this is how defensive wars against an overwhelming opponent are fought. Iran knows that they can't build an iron-clad air defense perimeter, there still isn't a reliable answer against stealth aircraft and cruise missiles. They never had a chance of shooting down every plane that enters their airspace, and that isn't their goal.

Instead, they will fight this war by absorbing blow after blow, hiding their capabilities and striking back when it is advantageous.

All Iran needs to do to win is:

1) Outlast the US air campaign - note this only requires protecting enough of their defensive capabilities to remain difficult. It does not require shooting down every US aircraft that enters their borders. It does not require shooting down most aircraft that enter their borders.

2) Prevent free shipping in the Strait of Hormuz.

That's it. They just need to apply economic pain as domestic and international opposition to the unprovoked attack grows.