I've come to believe the "why" of this is that many people don't think others have complete minds or experience a complete reality the same way that they do. As a side effect of this everything is contextualized based on how it personally affects them, or makes them feel. In this frame of reference most issues simply don't matter. Distant events are all just puppet theater.
This greatly eases life. Confronting the possibility that everyone else has a rich, inner experience is overwhelming. However, this is mental work that must be done. Children, by default, are very egocentric beings, but at some point growing adults should develop materialism the same way they must develop not shitting their pants. Some people never do this or perhaps they briefly peer over the edge, get overwhelmed, and retreat.
If you ask people, "So you think Bob down at the store doesn't have a complete mind?" they deny it since, when you put it that way, it sounds horrible. If you ask them, "So you think poor people in the DR Congo don't have a complete mind?" you may get a different answer. The farther from themselves another human is the more two-dimensional that "puppet" becomes.
I don't think this is unique to Americans anymore than children shitting their pants is. It's just that this behavior fractures the working class so the American system is happy to reinforce it. Unfortunately that's worked smashingly well. It's pretty bleak here. I do believe that people can change. I did. But there are overwhelming forces pushing dehumanization.