Danny was never a human.
While not exactly.
He kid of was, and kind of wasn't.
And that's not including the half-dead thing.
So during the start of Jack and Maddie's ghost hunting careers they experimented with the magic side of the world.
Then they stumbled across the ways to make something not living, alive.
And they thought 'well, this kind of involves our research, right?' Cos turning something that never lived but gets life most be some type of ghost stuff.
So they hunt down ways to make their experiment happen, from ancient texts to modern how to guides.
They checked it all.
Once their research was done, they made the body how the instructions say, with a few adjustments.
The only problem they had was that all of the recipes involved magic of some kind and neither Jack or Maddie had magic (they tested it multiple times)
So it was clear they had to get magic somehow.
And they did by stealing a magicians DNA, which was on him because he should know how dangerous it was to just leave DNA around, to be fair he probably wasn't expecting mad scientists to knock him out and take a blood sample.
And so it was added to the mixture of clay, their own blood,and the tiniest bit of ectoplasm moulded into a humanoid shape.
And finally it was the hard part.
Bringing it to life.
They took a page out of Victor Frankenstein's book and waited for a lighting storm, or in their case, a modified defibrillator.
The electricity acted as a spark to kick-start the process.
The blood formed the organs, skin, hair, and etc.
The ectoplasm breathed life into the golem, a bit too much.
Because once it was over they had a bouncing baby boy with the bluest eyes ever.
After running every test imaginable with each test confirming that the kid was human, they decided 'whelp, Jazz has a little brother now' and promptly forgot about it.
There was more exciting research to do after all.
When Danny's accident death happened it was only due to the magicians magic/ messed up luck powers and the ectoplasm in him that he survived.
Years go by and then Danny found out from reading their old notes.
His parents brushed it off saying that, yes, he was human and they loved him no matter what.
But Danny was a bit shook.
Finding out that he only existed not because his parents wanted him, but because they wanted to see if they could create.
Was he actually human? He was made out of clay for frick's sake!
And Danny handle this bombshell with grace and- he ran.
He ran as far as he could as fast as he could.
And so alone in a strange city and not willing to go back, he decided to seek out the magician who was technically his third parent.
What's the worst that could happen?
So off he went to find John Constantine.
Constantine: in my defense your father is a known polymorph rapist and I like alcohol. Although I thought he was strictly into young ladies
Diana: Constantine…
Constantine: You know what? We'll ask Shazam!
Diana who is not aware of the divine chat: ???