Because the small amount of alcohol is enough to get your brain into 'pi55ed' mode. Once it's in that mode, you can't think straight. The absurdity of your job and the people you work with becomes clear when seen through beer goggles. You realise that you should be back at the pub getting more pints in. Time goes more slowly and by five o'clock you just feel depressed.
That's why I never drink at lunchtime.
Oh dear jollygreen! I know the feeling. If I drink in the afternoon I have to go and sleep it off! And I am only 21 :( If i am going to treat myself to a cheeky bottle after lunch, I have to make sure I have nothing that needs doing, as it won't get done!