Serious answer, because in order for federal law to supersede state law they have to make their own law that "occupies the field" [1] or explicitly says in the law that it preempts state law.

[1] https://www.law.cornell.edu/wex/occupy_the_field

There are many state laws that contradict a federal law. But they have to actually be challenged by a federal court in other to be nullified. States can make up all the hair brained laws they want but if they never get sued by the feds then nobody cares.

For instance, ask the states legalizing marijuana right now in defiance of federal law.