Interesting idea.
I think the difference between cats and dogs in the UK is that cats are regularly seen as being outdoor animals, unlike dogs.
A dog on its own draws attention, and most people will accept a dog without an accompanying owner is going to be an escapee or a stray, and therefore at risk.
Not so with cats, who may be indoor only, indoor/outdoor, or wholly outdoors. In the US, cats have a great deal more natural predators than they do here in the UK. Many US owners have little option but to keep their cats as indoors cats, or to cat proof their gardens, if they want to keep predators away.
In the UK, cars and dogs are the biggest causes of loss of life in the feline populations. 47% of cats in the UK die from RTA's compared to just under 7% who succumb to viral diseases.
Not all cats will countenance wearing collars and not all people even bother to report a dead cat at the roadside - it's often viewed as an unfortunate fact of life for cats that aren't indoor only cats.
The vast majority of people finding a dead cat would never think of taking it to a vet so it could be scanned for a microchip, which is why I personally prefer my cats to wear safety collars. I've been fortunate enough to have two of my fatally injured cats returned because of the collar and ID tag.