JQL, in the UK you are assigned a number and you have your own plates made. As a result there are variations in the appearance of the plates. But in the USA you get the plates themselves from the same entity that assigned the numbers - they all come off the same assembly line. When you register your new-to-you vehicle, they give you the next plate in the stack they have there behind the counter, and put that into the computer on your record. They are metal stamped with the numbers and then painted with specific paints. They have exactly the same colors, same font, same kerning, same shape, etc, and then there are special reflective stickers on the corners that change each year. Which is not to say they couldn't be cloned - just that they're a bit outside of what any random sign shop would already have equipment to make. Hard to not have them look subtly wrong.
And since our traffic enforcement is still mostly done by live police officers instead of cameras, having subtly wrong plates would probably increase your chance of a traffic cop stopping you.
The more typical license plate crime you hear about, here, is people stealing the whole plate off a parked vehicle. Or cutting the corners off to take the little annual stickers.
I know a guy who had a green Camaro. One day he came out of his apartment to drive to work, and noticed his car had the wrong license plate number. He called the cops and it turned out that the plate on his car was registered to another green Camaro that had been reported stolen. He didn't know how long it had been on there - how often do you check that your license plate number hasn't changed? But it's probably good that he noticed it before a cop did.