Edit: so they don't use lidar... Is this a machine vision failure, then?
This is because almost everything in the environment is stationary to the car, so it is hard to distinguish between stationary things in your way and stationary things just off to the side.
So if the car in front of you slows down, it's obvious that you'll hit it if you don't slow down.
A car parked partially in your lane might just look like regular noise to the sensors. That is, using the current technology, if they (correctly) classified the parked car as an obstacle then they would (incorrectly) classify so many non-dangerous situations as obstacles so as to make the system infeasible; the car would emergency brake constantly as it drives down the road.
Some would say that until those obstacles can be classified correctly such technology should not be used at all, but I think it's probably doing more good than harm at the moment and hopefully a good solution can be found quickly. Even if they do fix this problem soon, I'm sure we'll hear plenty of stories where something goes wrong anyway, but hopefully they won't seem so obviously avoidable (who hits a parked car!)
Either having more than one sensor or using mirrors or something.
In fact, I'm sure that the camera systems in place are already doing this to some degree. I think it's more that it's not yet good enough at picking out these stationary objects that are in the path of travel.