I'm going to go out on a limb here and theorize that most schools, especially ivy leaguers, aren't exactly going to put out replicas of their diplomas.
Never know what kind of a seedy person would get their hands on it and try to pass it off as legit.
Might as well ask for blank driver's licenses and passports
According to this Yale website, you can obtain a special replica for filming purposes. It's not exactly like the real thing, but it'll look like every other one that's appeared on film.