A truck has a huge amount of kinetic energy and when accidents happen they are usuallly not pretty. Couple that with the fact that US trucks AFAIK are not required to have several safety features required elsewhere in the world, such as the underrun protection (metal bars) and that self-driving technology is far from proven at scale, maybe it's prudent to be a little bit careful.
Edit: after a quick Google, Embark Trucks[1] claims that they are the first to have driven 100k miles. That's a factor of 10 less than what Waymo claims to have driven. And there will be smaller outfits that will want to cut corners to get a piece of the pie.