|The short answer: Bigfoots are extremely rare and extremely cautious--so much so that the odds of a roadkill have not caught up with any yet.
The Roadkill Potential
Only a very small fraction of the thousands of credible sighting reports describe near-misses with vehicles. No substantiated reports describe a collision with a bigfoot.
Every other large mammal in North America is far more abundant than bigfoots. Hundreds of near-misses happen with other animals before an animal is hit. Some species are hit more often than others. Deer often get "dazzled in the headlights" and stand paralyzed in the paths of trucks. Bigfoots seem to be far more intelligent than that.
Those who have gotten closest to bigfoots say an analogy of "intelligent ape" is not as accurate as "hair-covered aboriginal man." Around humans their typical behavior is to flee or hide. They try to stay out of view or at least in the shadows when near people or moving vehicles.
Many roadside sightings describe them hanging back in the shadows of a tree line and waiting for a vehicle to pass before crossing the road. In almost all of those sightings a passenger spotted the figure first. Because of that, it's reasonable to extrapolate that a whole lot more lone drivers never notice when this behavior occurs, because a driver's attention is usually on the road ahead.
Waiting for a vehicle to pass before stepping out of the shadows to cross, merely demonstrates the same pattern of cautious behavior they exhibit in other encounters with humans.