All of the reasons above, plus sometimes the character just doesn’t have a name

When I’m talking about They Live, the protagonist is literally unnamed. Rowdy Roddy Piper is credited as “Nada” but even that is never used in the film (Great movie btw, go watch it if you haven’t seen it)

Then sometimes the actor is far more famous than the movie was, or the character wasn’t very memorable, or whatever the case is. Particularly true if the actor is appearing as a cameo