poorly constructed, no? always makes me ask "why would you want it if you can't (eventually) eat it."
It's a play on the word 'have'. You can have your cake means, you can get the cake, but that's not to say you can have it (i.e., enjoy it, get to eat it, too).
The saying, You cannot have your cake and eat it, too is another way of saying, you can't have the best of both worlds; it's either one or the other, but with cakes, one has to get the cake before one can enjoy/eat it.
There's also, You can have your cake and eat it, too. (You can have the best of both worlds: the cake is the prize, and getting to eat is the bonus!)
The saying dates back to the 1500's and was originally constructed in the reverse to say " can you eat your cake and then still have it left whole" [I](not the exact verbage however same idea), meaning if you've already eaten it you no longer have any left, hence you cannot have it both ways. either you eat it or you save it, it's one or the other but it cannot be both.