shipping - Why do address forms ask for the US state?

Inspired by Why do credit card forms ask for Visa, MasterCard, etc.? -

Why is it standard for web sites in the United States to ask for ZIP code and state?

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  Asking for the state is redundant. The ZIP code already provides an unambiguous identification of the address.

  
  


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  Asking for the state is inconvenient. It's usually a drop-down with 50 entries, and is often not caught by the browser's autofill feature (at least not in Chrome).

  
  


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  True, the state is basic required information when shipping with the USPS. Also, it may be relevant for tax and other regulations.

  
  

  However, it would be easy to fill in automatically based on the ZIP code the user entered, e.g. using USPS' API, or plain old Google Maps.

  
  

Why does everybody and their dog still force you to enter a state?