Some years back, some federal organization made new rules for semi-trailers with regard to the space between the road and the bottom of the trailer. These rules were designed with one problem in mind: decapitations due to rolling under the trailer. I think there was a rash of fatal wrecks that would have been non-fatal if the car was taller or the trailer was shorter, or something. I am under the impression that the rules are generally met these days by installing metal frames that stick down from the bottom of the trailer and prevent cars from fitting underneath.

Could it be that this trailer was out of spec? Could that be why 1. there is little damage to the truck, 2. The body is mostly intact but the roof is gone, and 3. the car did not detect the obstacle?

Depth estimation from cameras is challenging. I've seen recent research in generating depth maps using camera arrays, and even with precision camera calibration and state-of-the-art techniques, there are typically many glaring artifacts in the resulting depth map.

It might be that cameras introduce enough noise and artifacts into the mix that its not worth integrating them at this point.