Technically speaking, some states don't, at least not hard limits. For example, in Massachusetts, on roadways under MassHighway (well, now known as the MassHighway division of MassDOT) jurisdiction, speeds are supposed to be "reasonable and proper" (MGL Chapter 90, Section 17). Exceedance of posted speed limits are "prima facie" evidence of speeding, but not conclusive evidence, so it's technically possible to exceed the posted speed limit but not break the law if you can demonstrate that your speed is "reasonable and proper" (of course if you're not a lawyer or someone willing to spend a lot of money, this argument is not likely to fly in front of a magistrate). The MassPike and MassDCR roads require strict adherence to posted limits.
There are several other states that use "reasonable and proper" similar to MA, but I'd have to look them up to give a list. For the other states, I suspect the absolute limits are in place for simplicity.
Technically speaking, some states don't, at least not hard limits. For example, in Massachusetts, on roadways under MassHighway (well, now known as the MassHighway division of MassDOT) jurisdiction, speeds are supposed to be "reasonable and proper" (MGL Chapter 90, Section 17). Exceedance of posted speed limits are "prima facie" evidence of speeding, but not conclusive evidence, so it's technically possible to exceed the posted speed limit but not break the law if you can demonstrate that your speed is "reasonable and proper" (of course if you're not a lawyer or someone willing to spend a lot of money, this argument is not likely to fly in front of a magistrate). The MassPike and MassDCR roads require strict adherence to posted limits.
There are several other states that use "reasonable and proper" similar to MA, but I'd have to look them up to give a list. For the other states, I suspect the absolute limits are in place for simplicity.