Comment by SoftTalker

Back when McDonalds was first rolling out their standardized restaurants and menu, it was pretty common for restaurants to buy fresh ground beef from local sources, some of whom were less clean and less careful about temperatures and selling old meat. It was also common for them to mix in old meat with fresher meat to try to "hide" it. Since it's all ground up, it's hard to tell.

So they developed standards and demanded that all their hamburgers be formed, frozen, and packaged by the processor and kept frozen until cooking. It's really the safest way to handle that volume of ground beef through the distribution chain.

Apparently their 1/4 lb burgers now use fresh beef, not frozen. Not sure how that's handled but it sounds more risky.