I think we should also look at the role of those in authority in creating these, and most other 'celebrities'.
Is there really such a shortage of 'talent' that we must have the same celebrity featured in multiple shows and fields ?
I'm sure it must really annoy genuinely talented but unknown authors, or cooks, or whatever, when celebrities are given book contracts or television series not because their work is as good, but because their 'name' will help sales or viewing figures.
I don't know of any research on this, but my guess is that the celebrity circuit is just like the business executive delusion that talent is so rare it must receive excessive rewards: really, I would suggest, there's no shortage of ability, but those doing the employing are so unimaginative, risk-averse and profit-oriented, they'd rather choose somebody they already know, than give younger talent a break.
@GeofCox
The entertainment industry breaks younger folks in more ways than one. I saw abusers & their enablers get away with so much crap in my 10+ years in that industry. Higher ups protect the abusers - whether it's local or global environments. It's one main reason why I don't miss that industry. Anyone who tries to speak out/report abusers or confront enablers are ostracized/bullied - they try to silence people.