Why do sluggers make more than pitchers?

Big baseball contracts make no sense. A Rod gets millions more per year than Mariano Rivera but teams like the Red Sox and Cleveland (and indeed even the 1996 & 1998 Yankees) have proven over and over that pitching wins games and ultimately brings fans to the park. So what kind of sense is wasting so much money one one guy like A Rod (or any other big slugger) when you could be a much better team putting your etts in great pitching and good (as opposed to great) hitting?