In the early stage of a startup, companies have limited resources and a wide range of things that need to get done. Founders often wear ‘multiple hats’ to build and run the company. This follows to early employees—even with a stated job role, out of necessity they will need to go beyond it. This appeals to generalists (those who can do multiple kinds of things reasonably well) and is highly valued because the startup needs to do a wide range of things with limited resources.
Contrast that to a startup that is further along (> 1000 employees) and the needs change. Organization and administrative functions are required to coordinate many people working towards a shared set of goals. This puts pressure to hire the right person for a specific role where roles are defined to help administrators make sense of their workforce. Needs become more specific, and specialists are valued more because 1) they fit the definition of what’s needed more clearly 2) deeper expertise is required to solve more specific problems immediately.