Because it’s not covered by the Equal Employment Opportunity Act.
Saying that Law X exists answers the literal question, but not the implied actual question.
But of course, there could be laws that prohibit linking vehicle insurance cost to gender.
My solution, when I've been married, has been having my wife get insurance, and add me as a driver. But that doesn't matter so much now that I'm older.
Edit: I do agree that discrimination by gender is fundamentally unethical.