Because they can act?
This is how it goes:
- If you're a TV celebrity wanting to be known as a "proper" actor you play a minor role in a major West End show: something that gives you ten minutes on stage each night to remind people that you are actually a legitimate actor, albeit one who's been slumming it for a few years.
- If your career is on the wane and you never achieved any significant amount of fame then you're destined for a couple of seasons in Cats.
- If your career is on the wane and you used to be famous but you have no skill or experience in acting whatsoever then Chicago is the show for you: Roxie Hart, Mama Morton or Billy Flynn will suit you just fine.
- If nobody's heard of you for twenty years and most people think you are dead, then I'm afraid it's panto time.