That has to be the dumbest question of the day. Is your understanding of "science" based on watching the Big Bang Theory?
Fingerprinting is a science. But guess what, fingerprinting doesn't catch anyone before they break into the bank and steal the loot.
When the application of sound mental health science has concluded that a person is a danger to themselves or others, it makes sense to prohibit them from legally owing a firearm (which, unlike a knife or a bat has but one purposeful design - to kill.)
We prohibit felons from owning firearms because of their prior anti-social behavior. It would be ironic if we allowed firearms to be legally owned by someone who couldn't be convicted of a crime because they are so mentally incompetent that they can't tell right from wrong, and thus can't be convicted of a felony for killing someone else with that firearm.