Lions: Why do people insist on saying cubs?!?

Only the young of small cats are called kittens - domestic cats, Geoffroy's cats, margays, black-footed cats and so on. The correct name for the young of large cats like lions, tigers, leopards and so on is cubs. Whether or not they 'mew' or have fur or retractable claws or whatever has no bearing on anything. Just because a term is used for one animal or type of animal doesn't mean it can't be used for another - young rabbits, rats and others are also called kittens, for example, and young wolves, hyenas and others are also called cubs.