They're bad drivers because they drive badly. They make unsafe manoeuvres, they drive at unsafe speed, they create distracting environments, they disobey or don't even notice traffic signals, they drive while unfit, and so on.
The accidents are the _result_ no matter how you try to diminish them. The US record on motor car fatalities is particularly horrendous, as much as ten times worse per person than other economically similar countries in the same year. Many factors have a role to play, it's hard to untangle lax driver standards from the mix but they're in there.
On the ground civil agencies just work with the reality of society's decision. Bad drivers will be put behind the wheel of several tonnes of rolling metal, they will be too distracted to look where they're going for a few vital seconds and they will hit a truck. So we have to design the car and the truck so that when (not if) this happens the accident is as survivable as possible. That's why death rates have gradually fallen, the drivers remain just as bad, but their mistakes cost less lives. But it's a gross mis-statement to pretend that since we've re-shaped our whole society around this decision therefore people are good drivers.