It's all about the cost of a driver relative to the activity and risk; with taxis it's a much higher proportion than it is for aeroplanes.
Or use GPS to limit the speed of vehicles to the speed limit that's been set? For that matter why allow the sale of vehicles capable of exceeding 85 miles per hour (the highest speed limit anywhere in the country)?
To respond to your tangent: Breathalyzers are not all that reliable, nor is GPS localisation... And a national speed limit would mostly reduce revenue (not to mention the new problems regarding who is liable for maintaining the limit correctly). There is a great history of implementing practical safety requirements, maybe not the first globally, but when it becomes practical to adopt something we generally do.