Every human being is a mixture of DNA from their mother and father. Those strands are inseparable from one another.

However, we don’t have all of our mother and father’s DNA. Only pieces from each that make a full set when combined. We don’t have two full sets of DNA.

Neither does Steven.

Steven is not a full human with a full separate gem inside of him.

His gem is a part of him. When Rose was pregnant, bits and pieces of her “gem DNA” combined with pieces of Greg’s DNA. Just like we don’t inherit all of our parents’ DNA, Rose didn’t pass on all of her gemstone information. Some of those pieces were replaced by Steven’s human side.

Likewise, Steven didn’t gain a complete human body. Some of his human cells and systems require gem magic to function (“cells held together by magic”, as the crew calls it). This explains why Steven can shapeshift— his own cells are being held together by his light projection, and the projection can shuffle the cells around.

It makes complete sense that Rose didn’t come back for this reason:

If you took a human being and forcibly ripped them in half, separating all of their mom’s DNA from their dad’s, what would happen? Would their mom’s DNA literally be their mom, and their dad’s DNA be their dad?

Of course not.

You would end up with two incomplete sets of DNA— two halves of a person— struggling to reconnect.

And that’s exactly what happens when White Diamond removes Steven’s gem.

It’s a beautiful representation of how human beings create new life by a means of fusing their own DNA, and yet that person is inseparable and truly their own being.

And having a child, the human way... isn’t that what Rose wanted most?

Edit: yes, I know gems don’t technically have DNA, it’s a metaphor. Whatever magic biology stuff Rose gave him is essentially the stand-in for her DNA, at least from a narrative perspective. Not all of her gem information was passed on, just like not all of our human DNA makes it into our children.