First, our brain has a protector — the liver. The liver produces enzymes that help break down alcohol. At the same time, the liver has a processing speed limit: in one hour it can process 0.35 liters of beer, 0.15 liters of wine, or 0.4 liters of spirits. If you consume more than this amount, the liver does not have enough time to break down the alcohol, and it enters the bloodstream. This is what makes us drunk.
Second, even if alcohol is injected directly into brain cells, it will not destroy them. Alcohol disrupts the connections between cells rather than destroying the cells themselves.