(I did have to ask Jason for clarification as to why the boosters and fuel tank are visible but the orbiter itself is not, and as he explains in comments:
It's there, it's just hidden by the big gantry to the left of the main fuel tank and solid rocket booster. It encloses the shuttle as it sits on the pad, protecting it and giving the ground crews easy access to everything, then it swings out of the way as launchtime approaches. If you look on the ground around the pad, you can see a circular track - the moving gantry pivots on that.
You learn something every day! I figured it had to be something like that; obviously it would be kind of hard to get the orbiter mated to the booster rocket system when the rockets are already on the pad. But hey, you never know until you ask!)