I suspect Google got cold feet when they started considering the liability issues once this goes mainstream. Even their millions of miles road tests were done at low speeds near their campus.
In any passenger car, getting in a wreck at 100 km/h or above is more than likely a fatal event. "5 Star" safety ratings and the EU equivalent are only for 30~55 km/h crashes. Granted, even highway crashes involve some braking before impact, but the faster you go the more difficult it is to slow down enough to a 'safe' speed before impact.
Besides, the sensors on cars are more than capable of driving at 150 KM/H. While they cannot "see" as far as a human, they require much less time to process what they do see.
And then you get something like the adaptive cruise control on my BMW.
The bloody thing controls like a human just learning to drive - only focusing on what's right in front of it, instead of the general road situation, and making correspondingly jerky and drastic (albeit safe) maneuvers.
I often have to look ahead for it, and turn it off when I see a situation brewing that will entice it to drive like a moron. Then I handle that situation (typically by doing essentially nothing, e.g. just coasting for a few seconds), and turn it back on.