baffled beyond hopes

Since young, i have a relatively good command of the english language. However, i have been baffled by an english phrase or idiom. i always try to forget abt it, however, it comes back to me persistently for the past 20 years of my life.. when it cums back, i will bueh koon bueh jia... cuz very frustrating and makes me lan hur sio pua kor(balls fire burn pants) i.e du lan. And the shitty deal is tt it is used so often. it is more common than chicken rice and for sumthing so common, how to siam leh... and so, i decided that i have to face it like a ta por kia so i need help. anypne. juz explain the logic of this phrase to me and release me from the 2 decades of misery.. the phrase goes like this..

Siao lang Phrase: You cant have the cake and eat it too -_-

Why would limbei want the cake if limbei dun wanna eat it leh???
Assumptions made when deriving a logic for this ridiculous mad phrase.....
(i) limbei is a gan selfish person who dun believe in sharing
(ii) limbei dun believe in using cakes for ornamental purposes
(iii) limbei is gan hungry and limbei happen to love to eat cake

So guys help me out here thank you