Trucks in the USA

I'm not sure why heavy trucks are the way they are in the USA. If someone can explain then I'm all ears.

The most common type of truck I saw on my recent trip were semi-trailers. Almost all of the tractor units were conventional with hardly any cabovers. Is there a good reason why that's the case?

Then combinations were interesting. In California I'm not sure I ever saw a B-train or B-double rig, where the two trailers are each connected to a turntable. What I did see a lot of was a semi-trailer pulling a draw-bar trailer, and that draw-bar was often rising up from the front axle of the trailer, not perpendicular with the trailer body.

Then in most states I visited; aggregates were being transported on trucks that to my eye looked far too heavy to carry a useful payload. The trailers were often side dump or bottom dump, while others appeared to be transfer dumps such as the type in this video https://www.youtube.com/watch?v=woi0T4XVf1s

The trucks in the above video do appear far too heavy for a useful load of aggregate. Here's an Australian equivalent rig, I'm pretty sure this set-up delivers more in the one trip, correct me if I'm wrong https://www.youtube.com/watch?v=sYVT2_toS3I

In Nevada I saw road trains, but they too appeared to be heavy tare weights.