There are various reasons.
First, do you check the dates on the tags to know for sure they are empty but reserved? I ask that because around here, if you are looking for a campsite online for 7 days or more, the system may say there's nothing or just one site available. However if you arrive during the weekdays, you may see a lot of empty campsites with tags on them. If you read the tags, you'll see they are all for the upcoming weekend only. The host/ranger puts them out up to 2 weeks prior to the reservation so any folks who haven't booked in advance know how long the site is empty for their use. Online they don't show available because they are booked for part of the time frame you want.
In some states, they do not require campers to "occupy the campsite the first night of the reservation". So folks will book for an entire two weeks only to use the site on the weekends. They often do this to nab the site before the "reservation opening date" for holiday weekends.
Or there is no limit to how many nights you can stay, so they book the entire season to guarantee they have a site for the few days they decide to go camping.
Or they book multiple reservations way in advance and don't know which dates they can take off work until that month. So they just eat the other weeks or cancel at the last moment.
Or the host/ranger is lazy and doesn't pull cancelled reservation tags timely.
Higher penalties, limiting reservation changes, requiring first night occupation, limiting maximum lengths of stay, etc, are all ways parks try to reduce the empty sites. Some work some don't.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)