An AED isn't very effective at all.

Combined with proper CPR and alerting ALS, survival rates go way up. Studies are all over the board (because, as you rightly point out, causes of cardiac arrest are varied) but it's pretty generally accepted that CPR and an AED both more than double the patient's chance of survival, assuming prompt ALS response. It's huge when the AED is actually applied to a shockable rhythm. Even on its own, it's better than a bystander standing there with their hands in their pockets.

Even for professionals, manual defibrillation is getting rarer. Loads of ambulances have started stocking AEDs. Notably, healthcare professionals use a semi-automated form of AED, which gives them more control over the start/stop, but the actual shocking mechanism is identical.

They're not even designed to treat cardiac arrest

Ventricular tachycardia and fibrillation are both forms of cardiac arrest for which an AED is an effective intervention. If the person is actually in cardiac asystole (flatlined; clinically dead; heart no longer beating at all) indeed, the AED isn't going to do shit. However, if they've actually managed to somehow suddenly drop into asystole, ALS getting there, setting up, and applying intervention 10 minutes after they've dropped isn't going to be very successful either.

aortic fibrillation

You mean atrial? A-fib only really causes problems, to my knowledge, when it provokes other cardiac issues -- some of which are shockable rhythms by AED.

People in cardiac arrest are usually not brought back from an AED and or CPR alone

Not entirely correct; certainly true if they've slipped past arrest and into asystole. Both are effective interventions for bystanders to attempt before ALS arrives, and an AED stands as good a chance as manual fibrillation against its target rhythms.

take a First Aid/CPR class

Which will tell you to, if you have it, to apply that fucking AED. Or, more likely, to get another bystander to go run around and find an AED while you apply CPR.