An intrinsic part of the structure of what we call a “liberal arts education” is that students must pick an area – a “major” – in which to concentrate their studies, and in which they are expected to acquire some depth as well as breadth of knowledge. That’s true even at colleges like Brown and Amherst with an “open curriculum.” That means that they don’t have general education requirements everyone has to meet. It does not mean that students aren’t required to pick a major (or sometimes two) and to concentrate their studies that way.
However, as several people have pointed out, the liberal arts model allows students a fair amount of leeway – and indeed strongly encourages them – to take courses outside their majors.
Penn, by the way, also has a popular undergraduate PPE major.