[yakult]

Alternative hypothesis: most 'secret family recipes' are in fact original. However, the average SFR never get passed around, because it produces average-tasting stuff.

The recipe on food labels come from companies that know the product inside-out, have incentive to help you optimize the taste/effort tradeoff, and maybe have spent time and money on research. They tastes better, so they're the ones people remember.

tl;dr: people remember a disproportionately high number of plagiarized recipes because those are the good ones.