Signed Photos: Something I don't understand

I was just looking at my signed photos of Amber Benson, which I believe to be authentic because the person I got them off stood there and watched them being signed and is someone I know and trust. I started thinking about Certificates Of Authenticity (CoA). People sell photos and with them offer a CoA to assure the buyer that it is genuine. Why would people trust such a document? If someone has the wit, equipment and sociopathic attitude to fake an autographed photo then surely they could and would also fake a piece of laser printed paper with a signature to say that the associated photo is genuine. I've seen so many sales of obviously faked signed photos (the picture itself is obviously photoshopped, is of a type the supposed subject would be unlikely to sign (usually an actress's head on a body scanned from a porn mag) and the signature looks nothing like the other samples I have available which I know to be genuine) that have a CoA with them.

I can understand the desire to have signed photos, I have six in my Amber Benson Shrineon my walls. But, why trust a document that would be easier to fake than the item it claims to authenticate?