First, there are practical reasons. In some areas a parking lot may be required by zoning law. A parking lot at a bar provides employees who aren't drinking a place to put their cars so that every employee's car isn't parked along the street preventing other people in the neighborhood from parking their cars. If a fire or crime occurs police or emergency response can park closer to the bar which can aid in faster response time when seconds count.
Also note that in jurisdictions that have set a blood alcohol concentration limit drinking alcohol and then driving may not be considered illegal if the blood alcohol concentration is below the legal limit. Blood alcohol concentration can be affected by many factors such as age, weight and gender. Some people can drink more alcohol than others and still be under the legal limit.