Many readers may be well familiar with this, but a little review never hurts. Many liken sake to wine as it is not carbonated and has a similar alcohol content, others to beer since it is made from a grain. It is appreciated and consumed more like a wine, though. But no reason to split hairs, let's just allow it to have its own category, sake. Looking at how wine, beer and sake are brewed should make it more clear just what sake is.
Wine is a fermented beverage.
Fermentation is the process in which yeast converts sugar to alcohol and carbon dioxide, which in the case of wine, is allowed to escape. Sugars are already present in the grape, and these sugars are ready for use by the yeast cells as food and nutrients. Although this simple and short explanation does not do justice to the age-old art of making wine, it will serve our purposes here. Beer calls for another step. There is no fermentable sugar in barley grains, only long starch molecules that must be broken down into smaller sugar molecules, some that will ferment and some that will add to the flavor in other ways.