The idea is that you are not actually drunk but in some upset condition because of an emergency. And that condition might trigger the anti-drunk mechanism. A breathalyzer might work, but the drunk drive can always get someone else to breath into it. Also not sure how much calibration such device needs, can it last for a few months in a cold/hot car and still work correctly?

Once you want something to validate the breathalyzer (to avoid asking your child to start your car for you while you are drunk) it get complicated and more error prone.