Why? They just don't have the right look about them. Military men carry themselves a certain way that comes across in still photos.
Matt here is an exception. Who knows if he's still serving, but he certainly looks like he could have recently. I'll bet he was a Marine a.k.a. a jarhead.
He also looks rather nervous in some of these shots. For some reason I find that very alluring, perhaps because it suggests he'd be more easily seduced.
The originals for these photos are huge and total fifteen in number. They're archived online in a gallery here (mirror here) or you can download the entire set in a free zipped folder here.