How, as a bystander, can you be empirically certain that the motorist who sped past as you crossed the road was speeding, not wearing a seatbelt, drunk driving, and breaking the law?
You saw it, yes, but can you prove what you saw?
You can't? You're 'sure', but you have no hard evidence other than your eyes? My goodness.
We had better immediately dismantle all speeding laws, since citizens are not equipped with speed cameras in their heads. And we must abolish seatbelt laws too, since there is no way at all to police drivers and force them to comply every time they drive. And drink driving laws too, get rid, it's not like we can have a police officer present each and every time a driver enters a car, we'd never be able to police it.
We keep the speeding laws
We keep the seatbelt laws
We keep the drink driving laws
We can't police the moment of decision of a citizen to break each of these laws.
But we can punish those who are reported and caught, and we can use the methods available to us - certification, measurement and observation - to confirm without doubt that this was a transgression.
Sometimes, an innocent person is found not to have been stealing. Not to have been drunk. To be the genuine owner of the car.
This is not a failure of the law - this is a success. It demonstrates the law can distinguish between innocent and guilty.
Such laws are deterrents. And they work.
Yes, some people flout them. They gamble on not getting caught. But most are deterred for fear of consequences.
If the law shrugged, said, "but how can an average person be sure, tho, they simply can't be" and abolished speeding, drinking and seatbelt laws, the incidences of all of these would rocket. If the law said to people "please report any crime you think you've observed. But if you happen to be mistaken it's you for the dock" the cooperation between the public and law enforcement would disintegrate.
We need deterrents. We need consequences for those who are challenged for potentially breaking the law, and we need methods of determining the truth over what actually happened.
Birth certificates exist.
Sex can be observed and verified.
Women can be protected to eject and report potential transgressors, without threatened consequences for 'getting it wrong' and the potential transgressors can demonstrate, or not, their entitlement to that space. Or not.