Absolutely unsurprising. Iran has had literal decades to prepare for this fight. It's a big mountainous country, and they are arguably the most experienced tunnel builders in the world. If US intelligence says they still have half of their missile launchers, I would bet they have a lot more than that. They have also been watching and learning from the war in Ukraine, something the US has apparently failed to do.

Thousands of Iranian drones have yet to be destroyed

They could be producing thousands per day, so that statement is absolutely worthless.

Man-portable air defense systems (MANPADS) would not have been destroyed. However, the F-15 would probably have flown outside their range but not the A-10.

Launchers is one thing, but how about number of missiles to launch? It seems like they're only launching around 20/day. And they're mostly intercepted.