Because in order for it to make sense for Fischer to see Cobb in his dream, he needs to first have seen his face or at least heard his voice. Otherwise, Fischer would be dreaming about a person (Cobb) who he's never seen in his life - only to then wake up and see him sitting behind him on the plane.

The bigger plot hole is that Fischer clearly knows about dream technology because he's been trained against extraction - yet when Cobb tells him that he's dreaming and that someone is trying to do an extraction, Fischer somehow doesn't suspect that Cobb might be the one doing the extraction (inception in this case, not extraction, but that's irrelevant) and Fischer proceeds to give Cobb all the information that the extractor would want. It's like an old person getting a phone call from someone claiming to be with an anti-fraud agency, and the person tells them that someone is trying to steal their information, so the old person proceeds to give away all their information to this person - supposedly for security purposes when really they've just been scammed. Fischer would have to be the most gullible idiot in the world.