California gives out placards for permanent disabilities for free. Temporary disabilities, however, are $6 per instance. They last for six months. If you need to renew it, that’s another $6 for the renewal. The first disabled plate is free, provided you renew your registration (which is either $99 or free, depending), but after that it’s $20.
Additionally, while CA gives out permanent placards for permanent disabilities, other states don’t. Texas, for instance, only has permanent disability placards that last for two years. Even if they’re free, needing to renew it for something that’s not going to change is an unnecessary hassle.
And certification for service dogs does exist in the US; it’s just not handled by the government.
Another thing that should be considered: some people train their own service animals or get training done for their existing pet. If they already have a large, patient dog, it makes more sense to train that one than to get another dog (which can be $15,000-$50,000 depending on training). If someone trains their own dog, then the owner would most likely have trouble getting official recognition even if the dog meets all of the requirements. The government would probably require someone to get a certificate, which means only buying a dog that’s already been trained or getting a professional trainer to do it ($10,000-$30,000; cheaper but still unreasonable).
Not to mention that training a service animal, whether professionally or at home, requires the animal to be in public to make sure it can behave in uncontrolled environments. A law that requires government IDs would basically make it impossible to properly train the animal in public settings.
Also, the US only recognizes dogs as service animals, even though other animals can be useful, like mini horses that pull wheelchairs or monkeys that can grab things. If that’s not changed before an ID law is implemented, it’s likely that if a government ID is required for service animals, then service animals which are performing jobs but can’t get IDs because they’re not dogs would be banned.
So given all of that, pretty much the only benefit that people are arguing for is that people would be less dubious about service animals. Except that even then, that wouldn’t help as much because people would still claim that they’re fakers or that they forged the ID. We already see this with disability placards. The problem isn’t the animals; it’s that people discriminate against people who are visibly disabled. For instance, pushing someone’s wheelchair without consent because they’re “in the way” or even pushing someone out of a wheelchair because they want to prove the person is just faking it because they’re “too lazy to walk”. The wheelchair isn’t the problem; the problem is that some people are being bigots and questioning whether they really need their medical aid is a way of being bigoted without looking like you’re being a jerk to a disabled person. Requiring a special ID for a service animal is like requiring a special ID for a wheelchair; it’s not going to stop the bigots from claiming that it’s faked because that’s the fig leaf they’ve picked, and it’s going to keep a lot of people from getting help they need.
In short, implementing it would mean barely noticeable benefits to some people, while a bunch of others would get screwed over. Is that really worth it?