can you tell me why corn is number 1 dog food ?

I know the best dog food debate is intense.
However, in trying to decide on a food to use for my new dog I have come upon several unusual things. Everything I had read on nutrition agrees that pet food companies use corn because it is a cheap source of protein (compared to meat). I was not suprised to find corn as the first ingredient of the cheaper feed types but Science Diet also has corn as its main ingredient. I have a problem with that. I have found almost no one (professionally) who can give good feedback on this. Also Brewers rice which is listed on many nutrition sites as a filler. If you have corn and rice as you two main ingredients no wonder they have to add so many vitamins etc to have any nutritional effect.
Have any of you had similar problems. These are dogs not cows ! I want a commercial food that I can buy and not have to order. Dont want to pay more if quality is not there.

charlotte