Legislators decide things like that…

The limit has to be somewhere. Most experienced drivers can still drive reasonably safely after drinking small amounts of alcohol, e.g. one glass of beer; they can’t after drinking ten glasses of beer. So a limit had to be drawn somewhere. It’s different in different jurisdictions, for example AFAIK there are some countries in Europe (Czechia?) where the limit really is 0.0 or very close to that.