If you want to compare US westerns with SWs, you should only compare films which were made in the same time span. And there you already find enough cynicism or dirtyness or pessimism.
SWs were different, in parts very different from US westerns, but the US westerns of the 60s are often already different from a typical 50s western. The genre was always progressing from the beginning on, thematically as well as aesthetically.
In the 50 there was still a massive censorship, which just did not allow the kind of violence or nudity shown in the late 60s. The SWs of the late 60s would not have been possible to make in Italy 10 years before. That enough forbids to compare directly films of the 50s with films from 1965 onwards.
The comparison at the top of the thread in in parts true, in other parts not, it is a generalization (like stated), but the older US western was already a bunch of films too diverse to be accurately described without more differentiation. As a result the above comparison at the top of this thread mostly only reflects the usual cliches and is is neither really true for the US western nor for the SW.
For example there were always also US westerns portraying dirty, unshaved men, but it wasn’t done for the sake of itself. It became a style in 60s westerns, but if you check some US westerns you find it already there, before the Spags were shown in the USA.
And yes, there were already psychological westerns before 1960, there were pessimistic westerns, there were dirty looking westerns, there were quite violent scenes (comparatively), there were already westerns questioning the myths, and there was at least already one anti western in the 40s.
And actually SWs, even if that is too often claimed, are no anti-westerns, and are usually featuring no anti-heroes, but it still was a new kind of film and its protagonists were a new kind of hero. More violent, more selfish, but they still do the things the hero usually does.
And the hero of the 60s US westerns was not too far away from that. The real anti hero came in several 70s US westerns like Little Big Man, McCabe and Mrs Miller or Dirty Little Billy. There were also many westerns without any kind of hero, with just protagonists.
Well, it’s a complex subject, at least much more complex than it seems on first sight, I could write a book about it.