I agree about the other factors Dave, but health, emotional distractions and work-related stress are very difficult to detect and quantify. Alcohol, however, is simple to detect and quantify, and it's effects are well-recognised, so it seems to make sense to deal with it (even though the other issues you mention can't be dealt with so easily).

One has to start somewhere, does one not?

My comment "the only safe amount to drink when driving is....none" isn't an indication that I believe the legal limit should be 0 mg/ml - that would in itself create problems around detection and quantification (e.g. where someone had taken medication containing small quantities of alcohol). It was simply my own view and was my practice when I was a consumer of alcoholic beverages. Nobody liked a few beers more than I did but, when I was driving, it was strictly no booze for me.