The myth that searing the meat “seal in” its juices comes from a book by Justus von Liebig in 1847. The idea was that the crust would create a barrier that traps the meat juices inside.
Multiple experiments have shown that this is not the case. Harold McGee said on his 1984 book titled On Food and Cooking: “The crust that forms around the surface of the meat is not waterproof, as any cook has experienced: the continuing sizzle of meat in the pan or oven or on the grill is the sound of moisture continually escaping and vaporizing.”
High-end restaurants these days cook their steaks first, sealed in plastic, in low-temperature water baths, searing them only at the end in order to add flavor. The result is steaks that are juicier, moister, and more tender