Two reasons. First, after working for the state, I realized how easy it is to get the personal information behind a plate. Our office was secure to the max, but I could see a friend of a state worker gaining access to the information behind a plate. The information does not include SSN, etc., but it does include name, address, etc. So, if someone saw a vehicle with something inside they liked, they could come visit and take it when you're asleep. I've seen some interiors that are unbelievably impressive, and with impressive systems, and I wouldn't want to advertise the same. Sure, someone local can take it. But, why make it as easy for someone one state over (or, in the same state if they never saw the truck in person)?
Second, so a bum won't take it and call 1-800-(whatever PA's litter hotline is). Even if the person has to be a witness at the given issuance of the ticket, it's easier to be 100% sure it never occurs. Overprotective? Perhaps. But I prefer it that way.