Why does cooking meat at high heat seal in flavor?

This is actually a common myth. Cooking meat at high heat does not seal in juices or flavor — this idea was popularized in the 19th century by chemist Justus von Liebig and later spread by chef Auguste Escoffier, but it has been repeatedly disproven. What high heat actually does is trigger the Maillard reaction, a chemical process where amino acids and sugars on the surface of the meat react to create hundreds of new flavor compounds, producing that browned, savory crust. Seared meat actually loses moisture at a similar or even greater rate than meat cooked at lower temperatures. The appeal of high-heat cooking is the complex flavor and texture from browning, not any kind of moisture-sealing effect.