Simply put, licensing is a state issue and all states have education, experience, and examination requirements - so just because you've passed the exam (it's the same for all states) doesn't mean a specific state board is satisfied with the education and experience. Comity applications are essentially complete applications and no retest is necessary.
States have different requirements for education. Some strictly require ABET engineering degrees, some accept related science or technology degrees, some don't require a college degree at all. Alaska won't license any PE candidate until they have taken and passed an approved college-level course on Arctic Engineering.
States have different requirements for experience. Some require more years than others. Some require experience under PEs, others don't. Some require discipline-specific experience, others don't. Some require more references than others.
Finally, passing PE exam results do *not* necessarily transfer between states (although in practice they usually do). In theory, there are a number of issues that can prevent a passing PE exam result from State A from being honored in State B, for example:
1. NCEES offers many different PE exams, but states don't necessarily use them all. For example, NCEES offers a "Naval Architecture and Marine Engineering" PE exam, which you can pass for PE licensure in New York. But that doesn't mean you automatically qualify for PE comity in Arizona -- where that exam is not used.
2. States can offer their own PE exams, in addition to those offered by NCEES. For example, Oregon and Washington offer their own "Forest Engineering" exams, which you can pass for PE licensure there. But that doesn't mean you automatically qualify for PE comity in the District of Columbia, which only uses NCEES exams.
3. States can require supplements to NCEES exams. For example, California and Guam use the NCEES Civil PE exam, like all other jurisdictions. But they won't license any Civil PE candidate, even if they have already passed the NCEES Civil PE exam, until they have also passed a supplemental exam on seismic issues.
4. States can reject PE exam results -- even passing results -- if they were taken before meeting appropriate experience requirements. For example, suppose you qualify for the PE exam with 4 years of experience in State A, and pass it. Now you want comity in State B, which requires 6 years of experience. State B may reject the State A result, because it was obtained too "early", and make you retake the exam after you get 6 years of experience.