That's a good question. Probably because the press is lazy.
If the man and the woman in this relationship have no legal ties to each other, then the two children don't either.
But calling them step siblings is an easy shortcut to convey to the public that the 15 year old and the 10 year old have kind of, sort of a relationship between each other because of their respective parents' romantic and/or physical relationship.
In fact, I'd rate laziness as a sin of the press that's far more prevalent than outright bias.