I don't know. Part of me wants to say that Star Wars merch makes them so much money that they don't want to screw with the OT.
On the other hand, Disney bought Star Wars because they wanted to have something that appeals to boys the way Disney princesses appeal to girls. Despite this, they made a bunch of movies that pander to girls. So maybe they just can't help themselves.
Not officially a graduation, but we did have "Farewell Night" in 8th grade. It was more casual (no caps and gowns), but the same basic idea. This would have been the early 2000s.