If you have to use ALL alphanumeric chars - how can this possibly be brute forced quickly? If you're table of pins is stolen and the information readily connect

If you have to use ALL alphanumeric chars - how can this possibly be brute forced quickly? If you're table of pins is stolen and the information readily connects a user to a pin - that system is horrible from the jump. Human beings attempt to brute force their own pins, most of which have a greater insight into possible combinations and STILL get their account locked. Remember - 3 tries gets you locked. Why would it get broken quicker? If there's something I'm missing, I want to know it for the sake of being a better developer.