That standardizes the visual appearance, but not the semantics or issuance of the plates. EU residents are generally required to re-register a car when changing which country they reside in, which in turn has its own rules for what constitutes permanent residence.
And by comparison, all jurisdictions in the US and Canada have long agreed to a common set of physical/visual standards for license plates, despite variation between jurisdictions on the rules for registering vehicles.
I do love the variety of license plates in the US and Canada. My favorites are the bear shaped plate in NWT and the reproduction small plate in Delaware. Those are the only two I can think of that fall outside of the standard shape but are still.compatible with the screw hole pattern.
Fair, but there is some standard for issuance as well. VAT rules and certificates of conformity are EU wide.
VAT I guess being the interesting part given the original topic.