51% is the threshold for calling it “process cheese food.” The stuff that is called “process cheese” is only allowed additives off of a particular list: water, salt, milkfat up to 5% of the weight of the total, acidifying agents, spices, artificial coloring, mold inhibitors up to 0.2% or 0.3% of the total weight.

There’s basically not an easy way to make something match the legal definition of American cheese without making it out of at least 90% cheese, because the amount of water and fat you can add to fit within the requirement that the end result be 47% fat, except that only 5% of the total can be from added fat, makes it hard to cut corners.