That‘s literally an example of the decision depending on multiple factors. Separation of concerns -> more isolation -> stronger security of the overall system is exactly one of the possible benefits of microservices.
Scale is just one. There is also fault tolerance, security, organizational separation (which can be, up to a point, also be realized with libraries as you suggest), bigger ecosystem to choose from, …
Scale is just one. There is also fault tolerance, security, organizational separation (which can be, up to a point, also be realized with libraries as you suggest), bigger ecosystem to choose from, …