Simply put, no guardian has to protect the city and many don't. Take a look back at all the events your guardian has overcome and all the guardians we hear didnt. Gambit had guardians participating in illegal activities and even falling to the allure of the The original Dredgen Yor. Opulence had many guardians offer their services and allegiance to Calus to the extent of becoming his shadows or assassins. (Just a couple quick examples to help nurture that thought) The vanguard was originally created to give guardians a voice in the political squabbles of the city. One for each class so that there was fair representation for everyone. The vanguard is comprised of usually the higher end nominees from each class. For Titans this was saint-14 then Zavala. For warlocks this was osiris then Ikora rey. For hunters this was Talula Swiftrunner, Andaal Brask, then Cayde-6 with the current seat being vacant. I'm not saying the vanguard is always the greatest of the great but its hard not to respect any of the guardians who made that list and as such its no surprise that some would seek their guidance or accept their instructions as leaders of the last city. Additionally the vanguard also took up the goals of the Iron Lords, the original risen who used their powers to help rather than conquer. Its not until the city age that we we even refer to risen as guardians, because they primarily protect, or "guard" the last city and its un-powered civilians. The last point I have to address your curiosity is that risen/guardians don't "have" to do anything. The Traveler being self-aware of its godhood in the eyes of the people it supports, prevents it from directly contacting us or instructing us to do anything. The traveler provides but does not dictate. That's why the original risen were able to become cruel warmongering warlords rather than immediately starting to help humanity. The Traveler leaves its guardians with agency. I'd even argue we've had more lore and stories told of guardians falling and failing to uphold the beliefs of the vanguard and the city then we do of those who succeed on our moral scale. Even ghosts have their own agency and are not "forced" to do anything. They simply have an innate desire to find their partner and now during the city age, ghosts tend to want their guardians to join the last city so there is definitely some bias in the current telling of the story.