What's the ratio?
It's also probably 'cause you already need a masters to start a phd in Europe, while most of the programs in the states are essentially combined ms/phd programs. The actual number of courses ranges, and tends to depend on the school as much as anything else. Usually the first two or three years end up being courses, but I've heard of schools requiring anywhere from 30 to 60 credits worth of grad work (and there are probably schools that require more than 60 or less than 30 credits-it's really incredibly variable.)But they also make you finish the PhD in a few years, whereas you can take your time in the US.
Another data point which is probably outdated: when I was a grad student at Michigan in the late 1970s / early 1980s, my "core classes" amounted to two per semester for about the first five semesters. Three semesters of quantum, two of E&M, one each of classical mechanics and thermo/stat mech, and two math courses. After that, I think I had to take at least one class every two years (four semesters) until I finished my degree.