Think of it this way: It's easier to make new people than a PS, and there's a shorter wait for a new person after you...uh...place your order.
140 Million people are made each year. Only a few thousand private stocks were made in the last 25 years.
People are a dime a dozen compared to a PS.
Plus, PS guitars have serial numbers painted right on the darn things. We know where they were made. If you have one, you can call PRS and find out about it, just not get a certificate.
People don't come with numbers. Yet they have to be kept track of. Who knows where they were made? Even the parents don't always know...
"Did we make young Barbie on our vacation in the South of France or when we got home?"
"Um...I wasn't in the South of France with you."
"Oooops!"