It would be tough to standardize this across different vehicle models, much less manufacturers. A Model S isn't going to use the same battery pack as an electric heavy-duty pickup truck. You could use lots of small battery packs and bigger vehicles get more of them, but that complicates the machinery used to swap them.

It also hinders innovation. You can't produce batteries with innovative new form factors because nobody will be able to change them out at standard swap stations.

Finally, consumers would object to it. If I buy a brand new Model S I don't want to go to the swap station and give away my shiny new battery for someone's degraded high-mileage one that has an unknown probability of exploding or catching fire.