You usually need to show up in person to register a vehicle. If you intend to buy a car and drive it to a different state then you may not be able to register it in the origin state because most state stipulate that you can't register a vehicle if you do not reside there or have an address there (otherwise people in small shitty states would drive a couple hours to an adjacent state to save a few hundred on tax/registration/insurance). Registering it in the destination state would be similarly impossible without the title/bill of sale in hand and some states require an inspection prior to registration. Inspections stations need to be licensed. This means it's in some cases impossible to legally buy and drive a car bought private party because you can't register it where you buy it and you need have the car inspected before registering it where you're bringing it.
Depending on the states in question the fine for invalid registration is usually much less than the cost of having the unregistered vehicle towed from A to B so many people just slap an old plate on the back and cross their fingers.
Are there no exceptions for driving directly to a closest inspection place? In canton of Zurich, this is the AFAIK only case when you can drive without license plates (if you want to register your car and the DMV wants to inspect it, you can drive it plateless to DMV).
Depending on the states in question the fine for invalid registration is usually much less than the cost of having the unregistered vehicle towed from A to B so many people just slap an old plate on the back and cross their fingers.