I think it would take more development of Steemit to make it as user friendly as FB or Instagram to get celebrities to join. Being famous usually means being very busy and so unable to engage in the platform in any meaningful way. A YouTuber called @cinemasins joined for a bit but I think he gave up, having last posted a month ago. He didn't spend any time getting to know people and didn't offer exclusive content to Steemit folowers. He does great work on YouTube but it takes a while to find your feet in Steemit and people give up quickly.
I believe if you know a famous or seni famous person and can coach them in the way the community works so that they can train their followers ( sounding a bit like a pyramid scheme here lol) then they as a celebrity can do well on Steemit. It's still a very new platform and many people don't trust it because they don't understand it.