Wine is made by fermenting fruit juice with yeast, mainly Saccharomyces cerevisiae, and grapes are the standard raw material used worldwide because they naturally have the right balance of sugar and acidity for the yeast to work well. Cereals and millets are instead used to make beer or spirits, and that process needs an extra malting step to convert starch into sugar first. So when we talk about wine in the usual sense, it comes from grapes.