The reporting is that they were F35s though, not F15.
Iran's air defence system is based on the older Russian S300, which is incapable of detecting them.
Of course it wasn't that long ago that Iran was flying F14s and had complete air superiority over its neighbours. How the times have changed.
We also now know that the Israelis had informed the Iranians just before the attack through diplomatic channels of the impending attack, and that it would only contain specific military objectives.
Is the S300 just that useless? Possible. Did the Iranians decide to not respond and to "take one on the chin" in order to avoid a cycle of ever increasing conflict? Maybe. Had espionage already disabled Iran's air defence system? Also possible. We probably won't know for another 100 years.
Again, I think the most useful model here is Lebanon. Hezbollah has a desultory complement of SAM launchers, but no meaningful control over its airspace.
The Houthis were probably not far off from claiming to have shot down the Ingenuity chopper which NASA lost on Mars . In reality they can't shoot down a Cessna 172.