McDonald's in specific is the largest consumer of American beef in the country; there just aren't that many dairy cows to supply the demand.
FWIW, this is a response from McDonald's to an inquiry about their beef:
"McDonald's hamburgers are 100 percent pure domestic beef without fillers or seasonings. To maintain our customers' expectations and preference for lean, flavorful hamburgers, we carefully select fine cuts of grain fed beef and leaner cuts from dairy cattle. This is a typical combination for quality ground beef. More importantly, we have found that this combination results in a flavorful hamburger that also has a significantly lower percentage of fat (20 percent) than the government limits (30 percent).
"Contrary to your reader's letter, we never under any circumstances use waste or scraps. We use only select cuts of grain fed beef. Our ground beef suppliers are designated solely to McDonald's and their facilities are considered the most modern in the industry. In addition to meeting USDA requirements, our suppliers have worked closely with us to develop very strict, detailed specifications and requirements that are strictly enforced."