A very US-centric perspective. Whereas the folks in Europe do it in 3-4 years, come to the US and do a 2-3 year postdoc (with higher pay than a PhD student), and are ahead of their American peers.
Also, depending on where you do it in Europe, the pay as a PhD student is higher. At the extreme end, I knew students getting paid $60K/year in one country, while I was getting $24K/year in the US.
In some countries, the PhD program is fixed at 3 years. You either graduate by then, or you're out (in reality, they give some option for you to pay to continue, but almost no one can afford it). I suspect in those places, people have done a 2 year MS.
A US thing again? My friends all did 3 year bachelors, 3 year PhD. Some dragged out the PhD to 4. Those who do a masters do it in one year, and typically don't do a PhD. Some undergrad courses are 4 year and you get a masters at the end. And my UK bachelors was recognized as equivalent to a US masters degree for visa purposes.
Maybe this is CS-specific? Finishing physics PhD from high school in 6 years sounds just not enough time. Even exceptional people I know in my field needed at least 7-8 (3+4 or 3+2+3). 3 years into theoretical physics grad school is around the time people start doing decent research
