I think when you talk about someone like Julia Roberts you have to factor in "star quality": that mysterious "je ne sais quoi" that makes one person compelling to watch whereas another equallly talented person isn't. Julia Roberts is a perfect example: decent actress, pretty looks, but when she's onscreen there's a magnetic quality. A lot of celebs have that quality. A lot of lesser know actors may be equally talented (or more talented), just as good (or better) looking, but for whatever reason don't command the screen in the same way.
Of course, as people here have said, that "star quality" only goes so far. People want that magnistism in their movie, but without the skills to back it up with a good performance it can fall flat in the wrong role. Some celebs do have the chops to back it up, or even to turn it off if it serves the role. Some don't.