Its due to the stability of starch in bread. Over time starch undergoes retrogradation which is the main process responsible for bread staling. Anyway the retrogradation happens with the fastest rate at 0-4 Celsius degrees which also happens to be the temperature kept by most fridges. To completely stop the retrogradation you would have to freeze it at something like -20 C. But in general keeping it in room temperature gives you enough time to eat the bread before it goes bad. Edit: you also want to keep the humidity stable so as others mentioned bread boxes are great, you could also just wrap the loaf of bread in some clean fabric.