a lot of people get in there car and drive, they assume it is all ok as there mechanic has told them so,
That's why the auto makers and tire makers make the recommendations they do. There's also the financial incentive for the tire makers, and avoidance of liability for the auto makers (and the fact that most car makers put crappy tires on as OE).Safety is about dealing/planning for the lowest common denominator, i.e the nut behind the wheel.
As I said, most structural damage is visible upon close inspection, so for those of us who pay attention to our tires, going beyond 6 years isn't a significant risk. Not that my tires ever last that long before the tread is too low to suit me.