Why does adsense go for organic search engine rankings?

Why Adsense Publisher goes for Organic Search Engine Ranking. Once it is cheap, easy to access, and quiet targeted traffic can be available with hours. They why the publishers of adsense money making websites try their hard, day and night to get indexed in the google or yahoo or so big search engine? I think it is better to expend some cents by going through PPC and show the adsense sites to the targetted customers. What others suggests?

I am astonished by seeing this information. It in new to me that my competitor may arrange the situation by which google may cancel my account. So I will not go for competition. Better I shall go for only PPC advertisements. Thank You for reply.