grammar - Why is the British tv show "the Undatables" not called "the Undatable" without 's'?

There is a tv series on Channel 4 called "The Undatables" which I have just started watching.

The title keeps me wondering why it's not just called "the undatable" like the poor or the rich?

Last time I checked my grammar book, you only need an adjective after "the" to refer to a certain type of people.

Can you at least tell me why do you think the producer decided to make it plural by putting the "s" at the end?

Thanks