I've seen A LOT of posts about making HP with a 318-So why is it so ???

I've done several builds for people here with Desktop Dyno who've run real dyno time and found the results were very close. Now, mind you, I had all of the specs including head flow. The more info you input, the more accurate the output.
This is true. I do not know what very close is and to others, but the sims are not horrible unless your doing something very wacky with it. There getting better but... there really is nothing like an actual well calibrated & maintained dyno. More on this below.
I got a dyno sim, if you plug specs from builds you find it's in the ballpark, it's only strength to me is helping to narrow down cam choices and maybe heads to a lesser degree.
This is what I have said for a long time.
IMO, the best thing these dyno sims will do for YOU, is help teach you what various changes can do. I don’t think there a bad thing. I don’t think there a GREAT thing. I think they can help a good bit.

The best thing I think people will pick up from these dyno sims is how a cam change cam effect an engine output. Narrow or widen the centerline or increase/decrease a Cams duration on one side then both, you can get a idea of what can happen.

I do not know what the latest versions have upgraded, added or where they improved in. But there a run thing to have. I also don’t know what they currently assume of an intake manifold. Can I input individual runner flow numbers? Can it show the reduction in head flow due to the intake manifold. How about between manifolds?

There are limitations to the program. If anyone asks me about one, I’d just say enjoy the sim for what it is and play with it and learn something from it. There are few of them on the market. I don’t know who’s is best or worst. But I’ll just say I’ll never base my claimed power on it. Ether the actual dyno will or the drag strip will.
Because that’s where the fun is!