I'd like to know. Why people like myself who use Yahoo are subjected to endless Spam mailings from Lotteries?

On a daily basis I receive notifications from lotteries all over the world. Saying I've won in lotteries that I haven't even entered requesting additional information. Yea yea yea I know report them as Spam, but why bother? They continue to show up everyday even after reporting them! In my minds eye your Spam reporting system is a big crock! Because the more you report these people the more of the same is received. What's really going on? My latest was from a zipmail at YAHELL.. That possible could be the problem.....