Why did Adam and Eve have belly-buttons?

Sincerely, it's really the wrong question here. Instead you want to get to the place where you wonder something like: why is Eve from a 'rib'? What does that mean? That's the kind of question that is getting at the actual meaning of this. Stories in the Bible are not there to be mere concrete records, only history, only trivial. Instead, the stories are in the scripture for a purpose -- their meaning -- the truth they give us for our souls, to change us. Why 'rib'? -- because a rib is even closer to a person than their own skin, it's essential to protecting their very breath, and heart. This tells us the true, real quality of marriage. It's like your spouse is like a rib. If not, then you aren't all the way yet into the fullness of marriage you could be.

How do you know they did?

silly question .

Were they innies or outies?

Could you have asked a more trite question? They are fictional characters from a book of fables. You need to learn the difference between reality and fantasy.

To hold up their genitals.

Same reason as Annabeth Chase. She has a belly button. Adam and Eve didn't get them in the traditional way, but they need them just the same.

it is most likely that Adam and Eve DID NOT have belly buttons..... just because old time artists painted them with... does not mean they actually had them.... no one knows what they even looked like....

How do you know they had them?
Did you see their bellies perhaps?

Are you sure they did?

That ranks as the least of my concerns.

Mostlikely not

Good way of showing they had no parents