A European PhD is usually a minimum of 4 years nominally and the US PhD 3 years. That is longer. Also, most European universities demand a Master’s degree before you can start a PhD and in the US and UK you can start a PhD after a bachelor’s degree. If you fail the PhD, they give you a Master’s of Philosophy.
Europe = 3 or 4 years bachelor + 1 or 2 years Masters + 4 years PhD = 8 - 10 years.
USA: 3 + 3 =6 years.
In Europe many STEM or engineering degrees and medicine have a longer nominal study time than the arts, languages or psychology studies. 5 or 6 years instead of 4 years for a Masters degree in total.
Another issue is that European secondary school that prepares you for university is of a much higher academic standard than the US highschool. In the UK it is the A-level extra year that prepares you for uni.