Like I said, because the percent doesn’t change with the volume served. If you are an 1800s brewer you can calculate the ABV from samples, and subsequently sell kegs of various sizes, bottles, which in turn can be served in various amounts and the percent doesn’t change. And the industry never changed, nor the laws written. So it’s the way it is because that is how they used to do it and how laws were written and there hadn’t been a motivation for people to change that.