Because we have a spare one 
Not as heartless as it sounds, honest. She's a little elderly tortoiseshell who turned up in our garden a few months ago, very thin and tatty. We've been feeding her and housing her in our greenhouse ever since (she is now fatter and sleeker).
Despite registering her with local vets, Cats Protection League, lost and found pet websites etc, nobody has come forward to claim her. She's not chipped either.
Unfortunately she doesn't get on with our cats, nor they with her, so we can't keep her ourselves long-term. But she's a lovely little cat; really friendly, and quite happy with children - she even puts up with my two-year old trying to hug her. All she wants is food, fuss and somewhere warm to sleep.
PM me if interested!