Stereotypes don't come out of nowhere - you are correct. But they can oftentimes be the fruit of outdated, biased, or incorrect opinions with no connection to the truth. To clarify what I said above, if young Asian girls are stealing from your store more than any other group, the metrics back this up, and actions to prevent this are justified. If you don't have such data then that is where it becomes an issue.
While they may not apply to each and every individual, many times they do hold true to some extent, even if it may not be particularly pleasant to acknowledge.