because the more satisfying phrases are forbidden by convention (or the answerbank's puritanical approach to swearing) and you have to show surprise somehow.
Do you think people actually take it as a command? If that's true I am concerned that uninformed people who hear about "room to swing a cat in" will not realise this refers to cat-o-nine-tails and actually harm a few helpless puddy tats in order to estimate room sizes for housing surveys, describe holiday rentals, etc.