Because of google's site: command. The only way for the system to find out how many pages you have ads on is by running the google site: command. Otherwise your weight can't be established. It's a way of simplifying the system and make it work the same for all.
You don't because think about it... The system can not know the traffic to all these pages and it's not interesting because it's for link building so you don't need traffic stats. Traffic is a bonus IMO. Some people have sites specifically for this, not intended for people at all. Get crawled - get weight, aim it at their money sites. Others serve the ads on high volume traffic sites.