The issue is not what has happened in the past, but what could happen at the time. If the first officer became incapacitated then you are now single pilot with that operating pilot under the influence. Or what if the first officer was brand new on the line and the captain was intoxicated? There's several scenarios but just because there hasn't been a previous accident related to alcohol does not mean it's any less likely to happen than any other reason. Would you be happy if your family were down the back and it took off?