Well, to put it as briefly as possible, because visual imagination and sense of sight are qualitatively different things. They are not simply different in terms of degree; they are not the same at all. “Thinking about” my wife is a fluid thing; how she looks in my mind’s eye may be a function of how she looked when last I saw her, or, more often, a composite of the many, many ways she has looked since I have known her. My mind may capture a familiar facial expression, matched to a particularly nice hairstyle; it may imagine her in motion, speaking or laughing or doing, or from a collection of angles, or even as an abstraction: my wife, who is beautiful. These are not bad things; they are wonderful things, but they stimulate the brain in one kind of way.
Actually looking at her, via the medium of a photograph, is a different kind of stimulus that produces a different kind of response.
It is nice to have both options open to me. If I want the particular kind of happiness that comes from thinking about her, I can have that; if I want the particular kind of happiness that comes from looking at her.
I mean, I’ve eaten the cream of crab soup at my favorite seafood restaurant probably a dozen times in the last year or so. I know what it tastes like. I can sit here and think about the cream of crab soup all I want, and it is enjoyable to do so. But it is not the same experience as actually tasting the soup, in the same way as thinking about my wife or daughter is not the same as actually seeing them.
Also, pictures don’t just capture a face; they capture a particular moment in time. Can I think about what my wife looked like on our wedding day? Sure. But how accurate will my imagination really be? Since that day six years ago, she’s changed her hairstyle three times, and the color of her hair has undergone subtle natural changes. She gained and lost pregnancy weight, changing the contours of her face and body. Age has changed the look of her face. And the particular expression on her face at the moment our wedding picture was taken will probably never been seen again - similar smiles, maybe, but not that exact same expression. If I glance to my right, I can see my wife exactly as she was just after 1:00PM on our wedding day, see exactly what expression she had at that moment. And photos have a context; seeing that picture triggers associations, emotions, related thoughts - the flowers showed up late that day and my mother cursed out the delivery guy in the parking lot; Shannon’s always-proper ex-military Uncle got piss-ass drunk and danced like a crazy man; the filet was good; the champagne was lousy; how beautiful my wife’s hair looked from behind when I finally saw what she had done with it. I can “think about” these things, sure, but looking at that picture brings them all to me in one quick flood, brightening my day a little, and I can get back to work a little happier than before.
This is even more pronounced with my daughter. She no longer looks anything at all like she did just a year ago, when I first taught her how to sing “It’s a Hard Knock Life” from Annie, but there’s a picture of her from right around that time sitting under my computer monitor here at work that can bring that memory back to me in half a second.
That’s why. Because seeing and imagining are not the same, and there’s a place for both.