ELI5: Why does the U.S. not have a registration system for service dogs?

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If a handicapped driver needs to be able to park in a handicapped spot, they have to get a special tag or placard at the DMV. Dog owners already have to get yearly tags for their dogs with their county. Why isn’t there a national system that would register service dogs and give them a special tag or vest that designates them as a service dog?