One of the most important learnings a VC needs to have, ideally early in their career, is distinguishing the generational from the merely good.
This is an inherent advantage that someone who has worked inside of a Tier 1 venture firm, or someone who has worked inside one or multiple top tier startups, has. They have been exposed to the best companies, with the best teams, products, and processes. They have seen up close how the best founders carry themselves, how they communicate, hire & build teams, build products, and compete. They have seen world class metrics and world class execution. More importantly, they see these companies at an early stage and watch them evolve over time. There is arguably no better learning for discerning generational talent than by seeing it repeatedly up close.
Generational companies are so rare that it's possible for a VC to never be directly exposed to one. Without a proper frame of reference, it's hard for one's investment instinct to be sufficiently calibrated to know when they are in the presence of truly generational companies (it's highly likely they will experience many false positives).
It's possible to get this exposure through other means. Mike Moritz, for example, was a journalist, but had very close relationships with top founders and VCs, notably Steve Jobs. But the level of closeness he had with his subjects, as well as his natural instincts around talent identification, are unusually strong. For most, it's impossible to develop this frame of reference from the outside. It has to come from direct exposure.