Cause the creature’s name is NOT Frankenstein.
In the books (I know I know- boo, hiss), the Frankenstein Monster and his Bride were Frankies grandparents. Her dad was a green-skinned mad scientist named Viktor Stein, and he “created” her, thus- he’s her dad.
In the books they say that the monsters were just like people, living their lives and raising families and then when the movies were made, and they were portrayed as evil and terrifying and thats what ruined monster/human relations.
Personally, I think, as a light-hearted childrens line inspired by monsters, this take on it is acceptable. It’s kind of my head canon- Dr. Frankenstein created a family for the monster, they moved here, changed the name somewhat, put down roots, etc.
Just my two cents.