Missrandom, 'You cannot eat your cake and have it' was already listed in these exact words as a proverb in the 1500s, so clearly it was around even before that. Perhaps, therefore, you are misunderstanding a somewhat old-fashioned use of English. It simply means that once (a) - the eating of the cake - is completed, then (b) - still having it - is excluded. Pretty straightforward, really, and there is no suggestion that you cannot have the cake in the first place, as you imply.