Graduate degrees in the USA are more flexible than in the UK. Even at the PhD level, you are allowed to explore your academic interests and work in an interdisciplinary manner.
Whereas in the UK you are expected to focus on your subject from the very start of a master's or PhD, US universities aim to graduate well-rounded students, who can engage with the world in an interdisciplinary manner and with a broad skill set.
Master's and first or second-year PhD students often have the opportunity to take several courses outside of their department.
Some PhD programmes (especially in the humanities) require candidates to become proficient in another language, and most have professional development requirements such as:
- Teaching undergraduate classes
- Presenting at conferences
- Co-authoring academic papers with faculty
For this reason, in the USA master's degrees take two years and PhDs typically take four years to complete, to allow you room to explore your academic interests.