I don't think people are that incompetent, do you? Like if you met your former classmate at a high school reunion, what is the likelihood that person isn't who they say they are?
That is a different situation than what is going on here. Sure the likelihood that that person is who they say they are is pretty high, but what about some really famous person who no one has ever seen before (no pictures, nothing) and have only been known online and suddenly one day a random person says that he is this person? Would you really trust that guy to be saying the truth? Or would you ask for more proof of his identity? That is what we are doing here. No one knows who satoshi is, what he looks like, or anything about him except that he is some guy on the internet who created this cool thing called Bitcoin. Without any other information about Satoshi, no one can really vouch for him and we can't know that this person is him without some kind of proof since anyone can say "I am Satoshi".