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.
There are many state laws that contradict a federal law. But they have to actually be challenged by a federal court in order 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, all the states legalizing marijuana right now in defiance of federal law.
[1] https://www.law.cornell.edu/wex/occupy_the_field