Like many things its "the performers guild" - maybe called equity. Once you are in, you are in.

Just the same on soaps, where the same set of actors/actresses move around from show to show.

Helps if you have a relative in the business to start with.

It infuriates me that it extenRAB into quiz and reality shows.

Celebrity "Come Dine with Me"
Celebrity "Millionaires"
Celebrity "Family fortunes"
Celebrity "Strictly"
Celebrity "WifeSwap"

etc etc

Its obviously easier for the producers to manage celebs, than joe public.