You cannot get a PhD (in the US) without also passing an examination that, nominally at least, demonstrates you have broad and deep knowledge about your field. The qualifying exam is not the most important part, but it is a necessary part.
Some still exist, but this is less and less true over time. Even when a requirement is called an "exam", it isn't what a high-schooler would recognize as one.
Many of the exams described on this page are personalized and only require mastery of a handful of papers that really matter to the individual student, and knowledge of related work:
