Because it's Kimye, and between the two of them, they DON'T have two brain cells to rub together.
So therefore, they can't/don't see --- and wouldn't GIVE a damn, even if they COULD or DID! --- about the short-term AND long-range ridiculousness of that name for that child.
If I were her, were I blessed enough to make it to legal age, I'd change my name when I DID become a legal adult.
Then, I'd happily kick BOTH their asses from an amazing grace to a profound revelation!
OTOH, that IS their kid and the apple might not fall too far from the tree, so . . .
She might just opt to KEEP that name.