At the top levels, it's a lot more about where those units/buildings are than whether they exist. You know they're going to be mining and spending efficiently, so just seeing the number of workers gives you a very good idea of how much other stuff they have. Pros also check to see what technologies are being researched (buildings have a different animation while researching).

You can't get much more scouting than that because there is a cost associated with scouting, either because you lose a worker's time or because you spread out your army.