Coca Cola is always flat

I don't know whether it's our local shops such as checkers, spar, pick n pay but why on earth is every coke such as 1.5ltr and 2.25ltr always flat. It's either it's flat when you open it or after opened the next day it's flat? I mean we spending close to R28 for a bottle of coke I mean this is unacceptable we need answers as a country I can't be the only person.