European PhDs have better structure and rigid filters to weed out the unworthy dweebs. And the students take far less courses and generally work harder.

US PhD system is slow and often leads to counter-intuitive, interesting or just weird results, which may or mayn't pan out in the long run.

US PhD system has to be longer because

i) US undergraduate system is absolute trash (like worse than 3rd world 12th grader at their worst), specially in STEM, so the extra 2-3 years are essential.

ii) US students don't work that hard

iii) Quite a few STEM grad students are foreign students, sometimes from different undergard backgrounds than PhD topic, so they benefit from 1-2 years of background training and acclimitising to foreign land.