Because it is sealed against outside agents like bacteria and stuff before you open it. When you open it and sip from it you are introducing stuff from your mouth (which is loaded with bacteria) into the bottle. Leaving it at room temperature in a container filled with yummy food for the bacteria is bad since that is a perfect breeding ground for bacteria. The cold helps to slow the process so it stays edible longer, though not forever. Eventually it will go bad in the refrigerator too (believe me I know having cleaned out the frige at work several times). It's the same principle as meat. Leave it out for a day and it gets spoiled and nasty and rancid and the bacteria take over. Leave it in the fridge and it's good for quite a few days, and freeze it and it's good for a few weeks or more.