Do you really know that for a fact? I don't think one innocent person killed because someone was allowed to drive an unsafe hunk of shit on the roads.
That doesn't appear to be a complete sentence, but I'll attempt to address it.
In my anecdotal experience, which involves me driving at least 20k miles a year, I've never been in an accident caused by maintenance neglect on my behalf or someone else's. The only such instance I am aware of is a friend who had a friend who had a wheel bearing fail on him, but I'm not convinced any kind of reasonably labor/cost effective government-sponsored inspection system could have caught something like that anyway.
It's never been something that I've thought was necessary. Even if there were safety inspections, they couldn't possibly catch every potential issue, there are simply too many different modes of failure for it to make that significant of an impact. It would just be a needless beaurocratic money drain. I will say that police officers will pull people over if they see something obviously unsafe rolling down the road. There are certain standards, but they're addressed on the road by people who are already paid to keep the roads safe.
http://www.aa1car.com/library/auto_accident.htm
Mechanical failures are a factor in 12% to 13% of all auto accidents, according to all of the statistics I could find on the subject. In most cases, the mechanical failures can be attributed to normal wear or a lack of proper vehicle maintenance, not poor design or manufacturing defects (though there have been plenty of examples of the latter over the years).
Tire failure is considered a mechanical failure, so I wonder how many of these 13 percent of all accidents were caused by something other than a tire issue?
Most of those other failures are probably suspension/bushing/brake issues. Most people dont work on their own cars and will take their cars to mechanics, and mechancis are all too eager to get a customer to buy a brake job. Brakes are one area where you are generally very aware if you have a developing problem, to the point that people generally WILL address it for their own best interests. I dont think a government agent with a ruler is really necessary here.
So to me that leaves axle bearings, suspensions parts, and steering parts as being the most dangerous of the rest of the components in the car, and failures of those do happen of course... but I would still say they're fairly rare.