Why would they? There is literally no benefit to interviewers to do so -- they've already rejected you, so improving your performance won't help their company,

Why would they? There is literally no benefit to interviewers to do so -- they've already rejected you, so improving your performance won't help their company, and it could help their competitor that does hire you. It could cause problems ("would you have considered my interview more impressive if I was race/gender/nationality/height X? Time to lawyer up").