Question

Why is the OS called the manager of the computer system?​

Answer 1

Because that's fundamentally the job of an operating system: To arbitrate, abstract, manage, and allocate resources for the various programs running on the system. None of the programs handle any of that themselves, because that's the job of the OS.

Programs do not just start accessing memory all willy-nilly, they need to ask the OS to allocate them some, and how that allocation works is not the concern of the program, but the OS.