As for trucks having the same speed limit as cars in general: 1) a lot of the time there is a lower limit, 2) the truck itself has a lower max highway speed, 3) there a far fewer trucks on the road so it doesn't matter a much, 4) they are driven by professional drivers with things like electronically enforced daily driving limits, so many of the common causes of accidents are less likely.
As for trucks having the same speed limit as cars in general: 1) a lot of the time there is a lower limit, 2) the truck itself has a lower max highway speed, 3) there a far fewer trucks on the road so it doesn't matter a much, 4) they are driven by professional drivers with things like electronically enforced daily driving limits, so many of the common causes of accidents are less likely.