Becuase it is bought in sucjh vast quantities. I have never ever seen a barrel of beer, a bottle of spirits or a case of alcopops anywhere near out of date. The guys in head office phone fosters and say we will sell so many million of gallons is such a space of time and they work a deal out with them (and if the pubs don't order that many then they pay a higher rate for it). Also we used to give staff incentives to sell the right beers when we werent hitting the targets, so to make sure they were selling the right ones, we would make sure they mentioned Stella and Carling (and not Fosters and Kronie)when offering lager for example , and if they did mystery shopper would immediately hand that member of staff 50 quid.
I've been out this evening myself so my drinking may not explain this well.