Try producing a visual reproduction of a complete stranger you saw weeks ago.
It's bloody hard.
I've been in the position of trying to identify a thief who I partially apprehended, but he pushed past me partially knocking me down the stairs (no injuries) and ran off with my purse. The police drove me at breakneck speed to Central Birmingham Police station and actually showed me photos of men who they had stored images of. I think they were all thieves, but I am not sure of that.
I found a man that I was 99% certain was the man, but I was not prepared to swear it was him, because I am honest.
Had I had to draw him it would have been very difficult for me, despite my speciality being figure drawing and portraiture. It is not easy to remember features well enough to reproduce them even an hour later
However seeing images helps the mind.
I bet the Smiths were also shown images of men, but after the long delay it would be a miracle if they remembered the images of the man they saw, so fleetingly, to get a near perfect visual reproduction.