Why are US PhDs different from European PhDs?
First, let me start off with a counter-question:

Why would you expect them to be exactly the same?

Different regions have historically different educational systems on all levels, starting from Kindergarten. Why would you expect specifically the PhD degree to have a completely uniform definition everywhere in the world? Of course there are nowadays activities to make degrees (incl. PhDs) more comparable worldwide, but these things take time. As politicians in Europe have learned as part of the so-called Bologna process, you can't just top-down decide that from now on, we are using the US system.

Of course, there are sometimes reasonable arguments for differences in system. For instance, you concluded:


  it takes longer, on average, in the US than in Europe to finish a PhD since US PhD programs require less and have more coursework compared to Eur PhD programs.


In the US, a bachelor's degree is required for starting a PhD. In Europe, almost universally, you need a master or one of the older five-year diploma studies. So we in Europe expect students to hit the ground running basically from day 1 in their PhD. On the other hand, we don't require them to do much, or any, course work because they did all of that as part of the previous studies. Of course, if you then look only at the pure time spend in what is called the PhD studies, you end up with a shorter time in Europe.

Now you can of course go deeper down the rabbit hole and ask why European universities expect PhD students to have a master's degree first. The reason for that is mostly historical - around here, we often didn't even have Bachelor's degrees until the above-mentioned Bolognia process. What happened as part of this process was that decision makers ended up deciding that pretty much the first three years of the old diploma studies became "the bachelor" while the remaining two years became "the master". Of course, this reasoning led to the public opinion of somebody with "only" a bachelor's degree as a glorified college dropout. The universities implicitly also shared this notion, as there were never substantial motions to admit bachelor degree holders to PhD programmes in most universities. Slowly, the bachelor programmes are getting more profile as something better than just the first 3/5 of an actual degree programme, and consequently their public image also improves. Universities are nowadays also taking first tender steps towards making it easier for bachelor degree holders to start a PhD - however, so far, this is mostly targeted at making it easier for international students to enroll.

Important concluding remark: I am aware that a lot of the above contained pretty sweeping generalizations, which do not hold true everywhere. Specifically, Great Britain and Ireland already historically used a different system. However, I wanted to answer with something a bit more substantial than "systems are different everywhere".